"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/setter/associativity.wren

@@ -13,6 +13,6 @@ var c = Foo.new("c")
 
 // Assignment is right-associative.
 a.bar = b.bar = c.bar = "d"
-IO.print(a.toString) // expect: d
-IO.print(b.toString) // expect: d
-IO.print(c.toString) // expect: d
+System.print(a.toString) // expect: d
+System.print(b.toString) // expect: d
+System.print(c.toString) // expect: d

test/language/setter/instance.wren

@@ -2,7 +2,7 @@ class Foo {
   construct new() {}
 
   bar=(value) {
-    IO.print(value)
+    System.print(value)
   }
 }
 

test/language/setter/result.wren

@@ -4,4 +4,4 @@ class Foo {
 }
 
 var foo = Foo.new()
-IO.print(foo.bar = "value") // expect: result
+System.print(foo.bar = "value") // expect: result

test/language/setter/same_name_as_method.wren

@@ -1,7 +1,7 @@
 class Foo {
   construct new() {}
-  bar=(value) { IO.print("set") }
-  bar { IO.print("get") }
+  bar=(value) { System.print("set") }
+  bar { System.print("get") }
 }
 
 var foo = Foo.new()

test/language/setter/static.wren

@@ -1,6 +1,6 @@
 class Foo {
   static bar=(value) {
-    IO.print(value)
+    System.print(value)
   }
 }