"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/floor.wren

@@ -1,8 +1,8 @@
-IO.print(123.floor)      // expect: 123
-IO.print((-123).floor)   // expect: -123
-IO.print(0.floor)        // expect: 0
-IO.print((-0).floor)     // expect: -0
-IO.print(0.123.floor)    // expect: 0
-IO.print(12.3.floor)     // expect: 12
-IO.print((-0.123).floor) // expect: -1
-IO.print((-12.3).floor)  // expect: -13
+System.print(123.floor)      // expect: 123
+System.print((-123).floor)   // expect: -123
+System.print(0.floor)        // expect: 0
+System.print((-0).floor)     // expect: -0
+System.print(0.123.floor)    // expect: 0
+System.print(12.3.floor)     // expect: 12
+System.print((-0.123).floor) // expect: -1
+System.print((-12.3).floor)  // expect: -13