"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/module/cyclic_import/b.wren

@@ -1,9 +1,9 @@
 // nontest
-IO.print("start b")
+System.print("start b")
 
 var B = "b value"
-IO.print("b defined ", B)
+System.print("b defined " + B)
 import "a" for A
-IO.print("b imported ", A)
+System.print("b imported " + A)
 
-IO.print("end b")
+System.print("end b")