"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/timer/sleep_fibers.wren

@@ -3,17 +3,17 @@ import "timer" for Timer
 
 Scheduler.add {
   Timer.sleep(2)
-  IO.print("a")
+  System.print("a")
 }
 
 Scheduler.add {
   Timer.sleep(1)
-  IO.print("b")
+  System.print("b")
 }
 
-IO.print("main")
+System.print("main")
 Timer.sleep(3)
-IO.print("done")
+System.print("done")
 // expect: main
 // expect: b
 // expect: a