"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/field/closure.wren

@@ -11,6 +11,6 @@ class Foo {
 }
 
 var foo = Foo.new()
-IO.print(foo.closeOverGet.call()) // expect: Foo field
+System.print(foo.closeOverGet.call()) // expect: Foo field
 foo.closeOverSet.call()
-IO.print(foo.closeOverGet.call()) // expect: new value
+System.print(foo.closeOverGet.call()) // expect: new value

test/language/field/default_to_null.wren

@@ -1,6 +1,6 @@
 class Foo {
   construct new() {}
-  write { IO.print(_field) }
+  write { System.print(_field) }
 }
 
 Foo.new().write // expect: null

test/language/field/multiple.wren

@@ -10,11 +10,11 @@ class Foo {
   }
 
   write {
-    IO.print(_a)
-    IO.print(_b)
-    IO.print(_c)
-    IO.print(_d)
-    IO.print(_e)
+    System.print(_a)
+    System.print(_b)
+    System.print(_c)
+    System.print(_d)
+    System.print(_e)
   }
 }
 

test/language/field/nested_class.wren

@@ -3,19 +3,19 @@ class Outer {
 
   method {
     _field = "outer"
-    IO.print(_field) // expect: outer
+    System.print(_field) // expect: outer
 
     class Inner {
       construct new() {}
 
       method {
         _field = "inner"
-        IO.print(_field) // expect: inner
+        System.print(_field) // expect: inner
       }
     }
 
     Inner.new().method
-    IO.print(_field) // expect: outer
+    System.print(_field) // expect: outer
   }
 }
 

test/language/field/object_reference.wren

@@ -11,7 +11,7 @@ class Node {
       _left.write()
     }
 
-    IO.print(_value)
+    System.print(_value)
 
     if (_right is Node) {
       _right.write()

test/language/field/use_before_set.wren

@@ -1,6 +1,6 @@
 class Foo {
   construct new() {}
-  write { IO.print(_field) } // Compile a use of the field...
+  write { System.print(_field) } // Compile a use of the field...
   init { _field = "value" }  // ...before an assignment to it.
 }