"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/number/mod.wren

@@ -1,17 +1,17 @@
-IO.print(5 % 3) // expect: 2
-IO.print(10 % 5) // expect: 0
-IO.print(-4 % 3) // expect: -1
-IO.print(4 % -3) // expect: 1
-IO.print(-4 % -3) // expect: -1
-IO.print(-4.2 % 3.1) // expect: -1.1
-IO.print(4.2 % -3.1) // expect: 1.1
-IO.print(-4.2 % -3.1) // expect: -1.1
+System.print(5 % 3) // expect: 2
+System.print(10 % 5) // expect: 0
+System.print(-4 % 3) // expect: -1
+System.print(4 % -3) // expect: 1
+System.print(-4 % -3) // expect: -1
+System.print(-4.2 % 3.1) // expect: -1.1
+System.print(4.2 % -3.1) // expect: 1.1
+System.print(-4.2 % -3.1) // expect: -1.1
 
 // Left associative.
-IO.print(13 % 7 % 4) // expect: 2
+System.print(13 % 7 % 4) // expect: 2
 
 // Precedence.
-IO.print(13 + 1 % 7) // expect: 14
+System.print(13 + 1 % 7) // expect: 14
 
 // TODO: Unsupported RHS types.
 // TODO: Error on mod by zero.