"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/super/call_different_arity.wren

@@ -1,14 +1,14 @@
 class Base {
-  foo { IO.print("Base.foo") }
-  foo(a) { IO.print("Base.foo(a)") }
-  foo(a, b) { IO.print("Base.foo(a, b)") }
+  foo { System.print("Base.foo") }
+  foo(a) { System.print("Base.foo(a)") }
+  foo(a, b) { System.print("Base.foo(a, b)") }
 }
 
 class Derived is Base {
   construct new() {}
 
   foo(a) {
-    IO.print("Derived.bar(a)")
+    System.print("Derived.bar(a)")
     super
     super(1)
     super(1, 2)

test/language/super/call_other_method.wren

@@ -1,6 +1,6 @@
 class Base {
   foo {
-    IO.print("Base.foo")
+    System.print("Base.foo")
   }
 }
 
@@ -8,7 +8,7 @@ class Derived is Base {
   construct new() {}
 
   bar {
-    IO.print("Derived.bar")
+    System.print("Derived.bar")
     super.foo
   }
 }

test/language/super/call_same_method.wren

@@ -1,6 +1,6 @@
 class Base {
   foo {
-    IO.print("Base.foo")
+    System.print("Base.foo")
   }
 }
 
@@ -8,7 +8,7 @@ class Derived is Base {
   construct new() {}
 
   foo {
-    IO.print("Derived.foo")
+    System.print("Derived.foo")
     super.foo
   }
 }

test/language/super/closure.wren

@@ -9,4 +9,4 @@ class Derived is Base {
 }
 
 var closure = Derived.new().getClosure
-IO.print(closure.call()) // expect: Base
+System.print(closure.call()) // expect: Base

test/language/super/implicit_name.wren

@@ -1,6 +1,6 @@
 class Base {
   foo {
-    IO.print("Base.foo")
+    System.print("Base.foo")
   }
 }
 
@@ -8,7 +8,7 @@ class Derived is Base {
   construct new() {}
 
   foo {
-    IO.print("Derived.foo")
+    System.print("Derived.foo")
     super
   }
 }

test/language/super/indirectly_inherited.wren

@@ -1,6 +1,6 @@
 class A {
   foo {
-    IO.print("A.foo")
+    System.print("A.foo")
   }
 }
 
@@ -10,7 +10,7 @@ class C is B {
   construct new() {}
 
   foo {
-    IO.print("C.foo")
+    System.print("C.foo")
     super.foo
   }
 }

test/language/super/super_in_closure_in_inherited_method.wren

@@ -10,4 +10,4 @@ class B is A {
   construct new() {}
 }
 
-IO.print(B.new().callSuperToString) // expect: instance of B
+System.print(B.new().callSuperToString) // expect: instance of B

test/language/super/super_in_inherited_method.wren

@@ -8,4 +8,4 @@ class B is A {
   construct new() {}
 }
 
-IO.print(B.new().callSuperToString) // expect: instance of B
+System.print(B.new().callSuperToString) // expect: instance of B