"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_called_fibers.wren

@@ -1,21 +1,21 @@
 import "timer" for Timer
 
 var a = Fiber.new {
-  IO.print("a before")
+  System.print("a before")
   Timer.sleep(10)
-  IO.print("a after")
+  System.print("a after")
 }
 
 var b = Fiber.new {
-  IO.print("b before")
+  System.print("b before")
   a.call()
-  IO.print("b after")
+  System.print("b after")
 }
 
 // All fibers are suspended since they were directly called and not scheduled.
-IO.print("before")  // expect: before
+System.print("before")  // expect: before
 b.call()            // expect: b before
                     // expect: a before
                     // expect: a after
                     // expect: b after
-IO.print("done")    // expect: done
+System.print("done")    // expect: done