"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/divide.wren

@@ -1,12 +1,12 @@
-IO.print(8 / 2)         // expect: 4
-IO.print(12.34 / -0.4)  // expect: -30.85
+System.print(8 / 2)         // expect: 4
+System.print(12.34 / -0.4)  // expect: -30.85
 
 // Divide by zero.
-IO.print(3 / 0)         // expect: infinity
-IO.print(-3 / 0)        // expect: -infinity
-IO.print(0 / 0)         // expect: nan
-IO.print(-0 / 0)        // expect: nan
-IO.print(3 / -0)        // expect: -infinity
-IO.print(-3 / -0)       // expect: infinity
-IO.print(0 / -0)        // expect: nan
-IO.print(-0 / -0)       // expect: nan
+System.print(3 / 0)         // expect: infinity
+System.print(-3 / 0)        // expect: -infinity
+System.print(0 / 0)         // expect: nan
+System.print(-0 / 0)        // expect: nan
+System.print(3 / -0)        // expect: -infinity
+System.print(-3 / -0)       // expect: infinity
+System.print(0 / -0)        // expect: nan
+System.print(-0 / -0)       // expect: nan