"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/benchmark/fibers.wren

@@ -2,7 +2,7 @@
 var fibers = []
 var sum = 0
 
-var start = IO.clock
+var start = System.clock
 
 for (i in 0...100000) {
   fibers.add(Fiber.new {
@@ -12,5 +12,5 @@ for (i in 0...100000) {
 }
 
 fibers[0].call()
-IO.print(sum)
-IO.print("elapsed: ", IO.clock - start)
+System.print(sum)
+System.print("elapsed: " + (System.clock - start).toString)