"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/equality.wren

@@ -3,15 +3,15 @@ var b = 2..6
 var c = 2...5
 var d = 2...6
 
-IO.print(a == 2..5) // expect: true
-IO.print(a == 2..6) // expect: false
-IO.print(c == 2...5) // expect: true
-IO.print(c == 2...6) // expect: false
+System.print(a == 2..5) // expect: true
+System.print(a == 2..6) // expect: false
+System.print(c == 2...5) // expect: true
+System.print(c == 2...6) // expect: false
 
-IO.print(b != 2..5) // expect: true
-IO.print(b != 2..6) // expect: false
-IO.print(d != 2...5) // expect: true
-IO.print(d != 2...6) // expect: false
+System.print(b != 2..5) // expect: true
+System.print(b != 2..6) // expect: false
+System.print(d != 2...5) // expect: true
+System.print(d != 2...6) // expect: false
 
-IO.print(a != c) // expect: true
-IO.print(b != d) // expect: true
+System.print(a != c) // expect: true
+System.print(b != d) // expect: true