"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/logical_operator/and.wren

@@ -1,20 +1,20 @@
 // Note: These tests implicitly depend on ints being truthy.
-//       Also rely on IO.print() returning its argument.
+//       Also rely on System.print() returning its argument.
 
 // Return the first non-true argument.
-IO.print(false && 1) // expect: false
-IO.print(true && 1) // expect: 1
-IO.print(1 && 2 && false) // expect: false
+System.print(false && 1) // expect: false
+System.print(true && 1) // expect: 1
+System.print(1 && 2 && false) // expect: false
 
 // Return the last argument if all are true.
-IO.print(1 && true) // expect: true
-IO.print(1 && 2 && 3) // expect: 3
+System.print(1 && true) // expect: true
+System.print(1 && 2 && 3) // expect: 3
 
 // Short-circuit at the first false argument.
-IO.print(true) && // expect: true
-    IO.print(false) && // expect: false
-    IO.print(false) // should not print
+System.print(true) && // expect: true
+    System.print(false) && // expect: false
+    System.print(false) // should not print
 
 // Swallow a trailing newline.
-IO.print(true &&
+System.print(true &&
     true) // expect: true