"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/index.markdown

@@ -20,6 +20,6 @@ All Wren source files automatically have access to the following classes:
 * [Range](range.html)
 * [Sequence](sequence.html)
 * [String](string.html)
-* [IO](io.html)
+* [System](system.html)
 
 [embedding]: ../embedding-api.html