"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/if/else.wren

@@ -1,6 +1,6 @@
 // Evaluate the 'else' expression if the condition is false.
-if (true) IO.print("good") else IO.print("bad") // expect: good
-if (false) IO.print("bad") else IO.print("good") // expect: good
+if (true) System.print("good") else System.print("bad") // expect: good
+if (false) System.print("bad") else System.print("good") // expect: good
 
 // Allow block body.
-if (false) null else { IO.print("block") } // expect: block
+if (false) null else { System.print("block") } // expect: block