"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/core/range/iterate.wren

@@ -1,35 +1,35 @@
 // Inclusive.
 var range = 1..3
-IO.print(range.iterate(null)) // expect: 1
-IO.print(range.iterate(1)) // expect: 2
-IO.print(range.iterate(2)) // expect: 3
-IO.print(range.iterate(3)) // expect: false
-IO.print(range.iterate(4)) // expect: false
+System.print(range.iterate(null)) // expect: 1
+System.print(range.iterate(1)) // expect: 2
+System.print(range.iterate(2)) // expect: 3
+System.print(range.iterate(3)) // expect: false
+System.print(range.iterate(4)) // expect: false
 
 // Exclusive
 range = 1...3
-IO.print(range.iterate(null)) // expect: 1
-IO.print(range.iterate(1)) // expect: 2
-IO.print(range.iterate(2)) // expect: false
+System.print(range.iterate(null)) // expect: 1
+System.print(range.iterate(1)) // expect: 2
+System.print(range.iterate(2)) // expect: false
 
 // Negative inclusive range.
 range = 3..1
-IO.print(range.iterate(null)) // expect: 3
-IO.print(range.iterate(3)) // expect: 2
-IO.print(range.iterate(2)) // expect: 1
-IO.print(range.iterate(1)) // expect: false
+System.print(range.iterate(null)) // expect: 3
+System.print(range.iterate(3)) // expect: 2
+System.print(range.iterate(2)) // expect: 1
+System.print(range.iterate(1)) // expect: false
 
 // Negative exclusive range.
 range = 3...1
-IO.print(range.iterate(null)) // expect: 3
-IO.print(range.iterate(3)) // expect: 2
-IO.print(range.iterate(2)) // expect: false
+System.print(range.iterate(null)) // expect: 3
+System.print(range.iterate(3)) // expect: 2
+System.print(range.iterate(2)) // expect: false
 
 // Empty inclusive range.
 range = 1..1
-IO.print(range.iterate(null)) // expect: 1
-IO.print(range.iterate(1)) // expect: false
+System.print(range.iterate(null)) // expect: 1
+System.print(range.iterate(1)) // expect: false
 
 // Empty exclusive range.
 range = 1...1
-IO.print(range.iterate(null)) // expect: false
+System.print(range.iterate(null)) // expect: false