"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/static_field/closure.wren

@@ -11,6 +11,6 @@ class Foo {
 }
 
 Foo.initialize
-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/static_field/default_to_null.wren

@@ -1,5 +1,5 @@
 class Foo {
-  static write { IO.print(__field) }
+  static write { System.print(__field) }
 }
 
 Foo.write // expect: null

test/language/static_field/in_instance_method.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/static_field/multiple.wren

@@ -8,11 +8,11 @@ class Foo {
   }
 
   static 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/static_field/nested_class.wren

@@ -3,36 +3,36 @@ class Outer {
 
   static staticMethod {
     __field = "outer"
-    IO.print(__field) // expect: outer
+    System.print(__field) // expect: outer
 
     class Inner {
       construct new() {}
 
       static staticMethod {
         __field = "inner"
-        IO.print(__field) // expect: inner
+        System.print(__field) // expect: inner
       }
     }
 
     Inner.staticMethod
-    IO.print(__field) // expect: outer
+    System.print(__field) // expect: outer
   }
 
   instanceMethod {
     __field = "outer"
-    IO.print(__field) // expect: outer
+    System.print(__field) // expect: outer
 
     class Inner {
       construct new() {}
 
       instanceMethod {
         __field = "inner"
-        IO.print(__field) // expect: inner
+        System.print(__field) // expect: inner
       }
     }
 
     Inner.new().instanceMethod
-    IO.print(__field) // expect: outer
+    System.print(__field) // expect: outer
   }
 }
 

test/language/static_field/use_before_set.wren

@@ -1,5 +1,5 @@
 class Foo {
-  static write { IO.print(__field) } // Compile a use of the field...
+  static write { System.print(__field) } // Compile a use of the field...
   static init { __field = "value" }  // ...before an assignment to it.
 }