"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/list/grow_shrink.wren

@@ -8,4 +8,4 @@ for (i in 0..195) {
   list.removeAt(-1)
 }
 
-IO.print(list) // expect: [0, 1, 2, 3, 4]
+System.print(list) // expect: [0, 1, 2, 3, 4]

test/language/list/newlines.wren

@@ -6,19 +6,19 @@ var list = [
 
 ]
 
-IO.print(list[0]) // expect: a
-IO.print(list[1]) // expect: b
+System.print(list[0]) // expect: a
+System.print(list[1]) // expect: b
 
 // Newline after trailing comma.
 list = ["c",
 
 ]
 
-IO.print(list[0]) // expect: c
+System.print(list[0]) // expect: c
 
 // Newline in empty list.
 list = [
 
 ]
 
-IO.print(list.count) // expect: 0
+System.print(list.count) // expect: 0

test/language/list/trailing_comma.wren

@@ -1,4 +1,4 @@
 var list = ["a", "b",]
 
-IO.print(list[0]) // expect: a
-IO.print(list[1]) // expect: b
+System.print(list[0]) // expect: a
+System.print(list[1]) // expect: b