"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/for/truth.wren

@@ -6,27 +6,27 @@ class Iter {
 
 // False and null are false.
 for (n in Iter.new(false)) {
-  IO.print("bad")
+  System.print("bad")
   break
 }
 
 for (n in Iter.new(null)) {
-  IO.print("bad")
+  System.print("bad")
   break
 }
 
 // Everything else is true.
 for (n in Iter.new(true)) {
-  IO.print("true") // expect: true
+  System.print("true") // expect: true
   break
 }
 
 for (n in Iter.new(0)) {
-  IO.print(0) // expect: 0
+  System.print(0) // expect: 0
   break
 }
 
 for (n in Iter.new("")) {
-  IO.print("string") // expect: string
+  System.print("string") // expect: string
   break
 }