"IO" -> "System".

Get rid of the separate opt-in IO class and replace it with a core
System class.

- Remove wren_io.c, wren_io.h, and io.wren.
- Remove the flags that disable it.
- Remove the overloads for print() with different arity. (It was an
  experiment, but I don't think it's that useful.)
- Remove IO.read(). That will reappear using libuv in the CLI at some
  point.
- Remove IO.time. Doesn't seem to have been used.
- Update all of the tests, docs, etc.

I'm sorry for all the breakage this causes, but I think "System" is a
better name for this class (it makes it natural to add things like
"System.gc()") and frees up "IO" for referring to the CLI's IO module.

test/language/method/arity.wren

@@ -21,21 +21,21 @@ class Foo {
 }
 
 var foo = Foo.new()
-IO.print(foo.method) // expect: getter
-IO.print(foo.method()) // expect: no args
-IO.print(foo.method(1)) // expect: 1
-IO.print(foo.method(1, 2)) // expect: 3
-IO.print(foo.method(1, 2, 3)) // expect: 6
-IO.print(foo.method(1, 2, 3, 4)) // expect: 10
-IO.print(foo.method(1, 2, 3, 4, 5)) // expect: 15
-IO.print(foo.method(1, 2, 3, 4, 5, 6)) // expect: 21
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7)) // expect: 28
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8)) // expect: 36
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9)) // expect: 45
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)) // expect: 55
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)) // expect: 66
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)) // expect: 78
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)) // expect: 91
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)) // expect: 105
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)) // expect: 120
-IO.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)) // expect: 136
+System.print(foo.method) // expect: getter
+System.print(foo.method()) // expect: no args
+System.print(foo.method(1)) // expect: 1
+System.print(foo.method(1, 2)) // expect: 3
+System.print(foo.method(1, 2, 3)) // expect: 6
+System.print(foo.method(1, 2, 3, 4)) // expect: 10
+System.print(foo.method(1, 2, 3, 4, 5)) // expect: 15
+System.print(foo.method(1, 2, 3, 4, 5, 6)) // expect: 21
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7)) // expect: 28
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8)) // expect: 36
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9)) // expect: 45
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)) // expect: 55
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)) // expect: 66
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)) // expect: 78
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)) // expect: 91
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)) // expect: 105
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)) // expect: 120
+System.print(foo.method(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)) // expect: 136

test/language/method/empty_block.wren

@@ -3,4 +3,4 @@ class Foo {
   bar {}
 }
 
-IO.print(Foo.new().bar) // expect: null
+System.print(Foo.new().bar) // expect: null

test/language/method/long_name.wren

@@ -5,4 +5,4 @@ class Foo {
   }
 }
 
-IO.print(Foo.new().thisHasAMethodNameThatIsExactly64CharactersLongWhichIsTheMaximum) // expect: result
+System.print(Foo.new().thisHasAMethodNameThatIsExactly64CharactersLongWhichIsTheMaximum) // expect: result

test/language/method/many_methods.wren

@@ -2022,4 +2022,4 @@ result = result + foo.method996
 result = result + foo.method997
 result = result + foo.method998
 result = result + foo.method999
-IO.print(result) // expect: 1000
+System.print(result) // expect: 1000

test/language/method/newlines.wren

@@ -7,14 +7,14 @@ class Foo {
 var foo = Foo.new()
 
 // Allow newlines after commas and before ")".
-IO.print(foo.method("a",
+System.print(foo.method("a",
 
     "b"
 
     )) // expect: method a b
 
 // Allow newlines after commas and before "]".
-IO.print(foo["a",
+System.print(foo["a",
 
     "b"
 

test/language/method/operators.wren

@@ -21,19 +21,19 @@ class Foo {
 }
 
 var foo = Foo.new()
-IO.print(foo + "a") // expect: infix + a
-IO.print(foo - "a") // expect: infix - a
-IO.print(foo * "a") // expect: infix * a
-IO.print(foo / "a") // expect: infix / a
-IO.print(foo % "a") // expect: infix % a
-IO.print(foo < "a") // expect: infix < a
-IO.print(foo > "a") // expect: infix > a
-IO.print(foo <= "a") // expect: infix <= a
-IO.print(foo >= "a") // expect: infix >= a
-IO.print(foo == "a") // expect: infix == a
-IO.print(foo != "a") // expect: infix != a
-IO.print(foo & "a") // expect: infix & a
-IO.print(foo | "a") // expect: infix | a
-IO.print(!foo) // expect: prefix !
-IO.print(~foo) // expect: prefix ~
-IO.print(-foo) // expect: prefix -
+System.print(foo + "a") // expect: infix + a
+System.print(foo - "a") // expect: infix - a
+System.print(foo * "a") // expect: infix * a
+System.print(foo / "a") // expect: infix / a
+System.print(foo % "a") // expect: infix % a
+System.print(foo < "a") // expect: infix < a
+System.print(foo > "a") // expect: infix > a
+System.print(foo <= "a") // expect: infix <= a
+System.print(foo >= "a") // expect: infix >= a
+System.print(foo == "a") // expect: infix == a
+System.print(foo != "a") // expect: infix != a
+System.print(foo & "a") // expect: infix & a
+System.print(foo | "a") // expect: infix | a
+System.print(!foo) // expect: prefix !
+System.print(~foo) // expect: prefix ~
+System.print(-foo) // expect: prefix -

test/language/method/static.wren

@@ -7,7 +7,7 @@ class Foo {
   static bar(arg) { "on metaclass " + arg }
 }
 
-IO.print(Foo.new().bar)        // expect: on instance
-IO.print(Foo.bar)              // expect: on metaclass
-IO.print(Foo.new().bar("arg")) // expect: on instance arg
-IO.print(Foo.bar("arg"))       // expect: on metaclass arg
+System.print(Foo.new().bar)        // expect: on instance
+System.print(Foo.bar)              // expect: on metaclass
+System.print(Foo.new().bar("arg")) // expect: on instance arg
+System.print(Foo.bar("arg"))       // expect: on metaclass arg

test/language/method/subscript_operators.wren

@@ -9,9 +9,9 @@ class Foo {
 }
 
 var foo = Foo.new()
-IO.print(foo["a"]) // expect: 1-subscript a
-IO.print(foo["a", "b"]) // expect: 2-subscript a b
-IO.print(foo["a", "b", "c"]) // expect: 3-subscript a b c
-IO.print(foo["a"] = "value") // expect: 1-subscript setter a = value
-IO.print(foo["a", "b"] = "value") // expect: 2-subscript setter a b = value
-IO.print(foo["a", "b", "c"] = "value") // expect: 3-subscript setter a b c = value
+System.print(foo["a"]) // expect: 1-subscript a
+System.print(foo["a", "b"]) // expect: 2-subscript a b
+System.print(foo["a", "b", "c"]) // expect: 3-subscript a b c
+System.print(foo["a"] = "value") // expect: 1-subscript setter a = value
+System.print(foo["a", "b"] = "value") // expect: 2-subscript setter a b = value
+System.print(foo["a", "b", "c"] = "value") // expect: 3-subscript setter a b c = value

test/language/method/too_many_arguments.wren

@@ -1 +1 @@
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17) // expect error
+System.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17) // expect error