"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.

doc/site/core/bool.markdown

@@ -9,10 +9,9 @@ Boolean values. There are two instances, `true` and `false`.
 
 Returns the logical complement of the value.
 
-    > !true
-    false
-    > !false
-    true
+    :::dart
+    System.print(!true) // "false".
+    System.print(!false) // "true".
 
 ### toString