"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/map/key_iterate.wren

@@ -4,23 +4,23 @@ var a = {"one": 1, "two": 2, "three": 3, "four": 4}.keys
 // entry table and the hashing process isn't specified. So we just validate
 // what we can assume about them.
 
-IO.print(a.iterate(null) is Num) // expect: true
-IO.print(a.iterate(null) >= 0) // expect: true
+System.print(a.iterate(null) is Num) // expect: true
+System.print(a.iterate(null) >= 0) // expect: true
 
-IO.print(a.iterate(0) is Num) // expect: true
-IO.print(a.iterate(0) > 0) // expect: true
-IO.print(a.iterate(1) is Num) // expect: true
-IO.print(a.iterate(1) > 0) // expect: true
-IO.print(a.iterate(2) is Num) // expect: true
-IO.print(a.iterate(2) > 0) // expect: true
-IO.print(a.iterate(3) is Num) // expect: true
-IO.print(a.iterate(3) > 0) // expect: true
+System.print(a.iterate(0) is Num) // expect: true
+System.print(a.iterate(0) > 0) // expect: true
+System.print(a.iterate(1) is Num) // expect: true
+System.print(a.iterate(1) > 0) // expect: true
+System.print(a.iterate(2) is Num) // expect: true
+System.print(a.iterate(2) > 0) // expect: true
+System.print(a.iterate(3) is Num) // expect: true
+System.print(a.iterate(3) > 0) // expect: true
 
 var previous = -1
 var iterator = a.iterate(null)
 while (iterator) {
-  IO.print(iterator > previous)
-  IO.print(iterator is Num)
+  System.print(iterator > previous)
+  System.print(iterator is Num)
   previous = iterator
   iterator = a.iterate(iterator)
 }
@@ -38,8 +38,8 @@ while (iterator) {
 // expect: true
 
 // Out of bounds.
-IO.print(a.iterate(16)) // expect: false
-IO.print(a.iterate(-1)) // expect: false
+System.print(a.iterate(16)) // expect: false
+System.print(a.iterate(-1)) // expect: false
 
 // Nothing to iterate in an empty map.
-IO.print({}.keys.iterate(null)) // expect: false
+System.print({}.keys.iterate(null)) // expect: false