"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/limit/many_globals.wren

@@ -4095,7 +4095,7 @@ var global4094 = 4094
 var global4095 = 4095
 var global4096 = 4096
 
-IO.print(global4096) // expect: 4096
+System.print(global4096) // expect: 4096
 
 // Symbol tables currently use O(n) to look up names. This makes this test
 // painfully slow when a large number of names are defined. This is a
@@ -65484,5 +65484,5 @@ var global65472 = 65472
 // Not defining the full 65,536 here because the core library also defines some
 // global names.
 
-IO.print(global65472) // expect 65472
+System.print(global65472) // expect 65472
 */