"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/nonlocal/assignment.wren

@@ -1,7 +1,7 @@
 var Nonlocal = "before"
-IO.print(Nonlocal) // expect: before
+System.print(Nonlocal) // expect: before
 Nonlocal = "after"
-IO.print(Nonlocal) // expect: after
+System.print(Nonlocal) // expect: after
 
 class Foo {
   static method {
@@ -10,9 +10,9 @@ class Foo {
 }
 
 Foo.method
-IO.print(Nonlocal) // expect: method
+System.print(Nonlocal) // expect: method
 
 Fn.new {
   Nonlocal = "fn"
 }.call()
-IO.print(Nonlocal) // expect: fn
+System.print(Nonlocal) // expect: fn

test/language/nonlocal/in_block_scope.wren

@@ -2,7 +2,7 @@ var Nonlocal = "outer"
 
 {
   var Nonlocal = "inner"
-  IO.print(Nonlocal) // expect: inner
+  System.print(Nonlocal) // expect: inner
 }
 
-IO.print(Nonlocal) // expect: outer
+System.print(Nonlocal) // expect: outer

test/language/nonlocal/mutual_recursion.wren

@@ -8,5 +8,5 @@ class Bar {
   static foo { Foo.new() }
 }
 
-IO.print(Foo.bar) // expect: instance of Bar
-IO.print(Bar.foo) // expect: instance of Foo
+System.print(Foo.bar) // expect: instance of Bar
+System.print(Bar.foo) // expect: instance of Foo

test/language/nonlocal/nonlocal_in_initializer.wren

@@ -1,2 +1,2 @@
 var A = A == null
-IO.print(A) // expect: true
+System.print(A) // expect: true

test/language/nonlocal/nonlocal_without_initializer.wren

@@ -1,2 +1,2 @@
 var A
-IO.print(A) // expect: null
+System.print(A) // expect: null

test/language/nonlocal/null_before_defined.wren

@@ -1,3 +1,3 @@
-IO.print(Foo) // expect: null
+System.print(Foo) // expect: null
 var Foo = "value"
-IO.print(Foo) // expect: value
+System.print(Foo) // expect: value

test/language/nonlocal/undefined.wren

@@ -1,6 +1,6 @@
 var fn = Fn.new {
-  IO.print(Foo)
-  IO.print(Bar)
+  System.print(Foo)
+  System.print(Bar)
 }
 
 // expect error line 6

test/language/nonlocal/use_in_function.wren

@@ -1,5 +1,5 @@
 var Global = "global"
 
 Fn.new {
-  IO.print(Global) // expect: global
+  System.print(Global) // expect: global
 }.call()

test/language/nonlocal/use_in_function_before_definition.wren

@@ -1,5 +1,5 @@
 var f = Fn.new {
-  IO.print(Global)
+  System.print(Global)
 }
 
 var Global = "global"

test/language/nonlocal/use_in_method.wren

@@ -4,11 +4,11 @@ class Foo {
   construct new() {}
 
   method {
-    IO.print(Global)
+    System.print(Global)
   }
 
   static classMethod {
-    IO.print(Global)
+    System.print(Global)
   }
 }
 

test/language/nonlocal/use_in_method_before_definition.wren

@@ -2,11 +2,11 @@ class Foo {
   construct new() {}
 
   method {
-    IO.print(Global)
+    System.print(Global)
   }
 
   static classMethod {
-    IO.print(Global)
+    System.print(Global)
   }
 }