"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/this/closure.wren

@@ -5,4 +5,4 @@ class Foo {
 }
 
 var closure = Foo.new().getClosure
-IO.print(closure.call()) // expect: Foo
+System.print(closure.call()) // expect: Foo

test/language/this/nested_class.wren

@@ -2,16 +2,16 @@ class Outer {
   construct new() {}
 
   method {
-    IO.print(this) // expect: Outer
+    System.print(this) // expect: Outer
 
     Fn.new {
-      IO.print(this) // expect: Outer
+      System.print(this) // expect: Outer
 
       class Inner {
         construct new() {}
 
         method {
-          IO.print(this) // expect: Inner
+          System.print(this) // expect: Inner
         }
         toString { "Inner" }
       }

test/language/this/nested_closure.wren

@@ -5,4 +5,4 @@ class Foo {
 }
 
 var closure = Foo.new().getClosure
-IO.print(closure.call().call().call()) // expect: Foo
+System.print(closure.call().call().call()) // expect: Foo

test/language/this/this_in_method.wren

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

test/language/this/this_in_static_method.wren

@@ -1,7 +1,7 @@
 class Foo {
   static test {
-    IO.print(this == Foo) // expect: true
-    IO.print(this.bar) // expect: bar
+    System.print(this == Foo) // expect: true
+    System.print(this.bar) // expect: bar
   }
 
   static bar { "bar" }