"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/inheritance/is.wren

@@ -11,12 +11,12 @@ var a = A.new()
 var b = B.new()
 var c = C.new()
 
-IO.print(a is A) // expect: true
-IO.print(a is B) // expect: false
-IO.print(a is C) // expect: false
-IO.print(b is A) // expect: true
-IO.print(b is B) // expect: true
-IO.print(b is C) // expect: false
-IO.print(c is A) // expect: true
-IO.print(c is B) // expect: true
-IO.print(c is C) // expect: true
+System.print(a is A) // expect: true
+System.print(a is B) // expect: false
+System.print(a is C) // expect: false
+System.print(b is A) // expect: true
+System.print(b is B) // expect: true
+System.print(b is C) // expect: false
+System.print(c is A) // expect: true
+System.print(c is B) // expect: true
+System.print(c is C) // expect: true