"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/map/grow_and_shrink.wren

@@ -64,13 +64,13 @@ for (fish in fishes) {
   map[fish] = fish.count
 }
 
-IO.print(map.count) // expect: 249
+System.print(map.count) // expect: 249
 
 for (n in 0...150) {
   map.remove(fishes[n])
 }
 
-IO.print(map.count) // expect: 99
+System.print(map.count) // expect: 99
 
 // Make sure we can still find all of the remaining ones.
 var contained = 0
@@ -78,4 +78,4 @@ for (n in 150...249) {
   if (map.containsKey(fishes[n])) contained = contained + 1
 }
 
-IO.print(contained) // expect: 99
+System.print(contained) // expect: 99