"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/string/iterate.wren

@@ -1,29 +1,29 @@
 var s = "abçd"
-IO.print(s.iterate(null)) // expect: 0
-IO.print(s.iterate(0)) // expect: 1
-IO.print(s.iterate(1)) // expect: 2
+System.print(s.iterate(null)) // expect: 0
+System.print(s.iterate(0)) // expect: 1
+System.print(s.iterate(1)) // expect: 2
 // Skip 3 because that's the middle of the ç sequence.
-IO.print(s.iterate(2)) // expect: 4
+System.print(s.iterate(2)) // expect: 4
 // Iterating from the middle of a UTF-8 sequence goes to the next one.
-IO.print(s.iterate(3)) // expect: 4
-IO.print(s.iterate(4)) // expect: false
+System.print(s.iterate(3)) // expect: 4
+System.print(s.iterate(4)) // expect: false
 
 // Out of bounds.
-IO.print(s.iterate(123)) // expect: false
-IO.print(s.iterate(-1)) // expect: false
+System.print(s.iterate(123)) // expect: false
+System.print(s.iterate(-1)) // expect: false
 
 // Nothing to iterate in an empty string.
-IO.print("".iterate(null)) // expect: false
+System.print("".iterate(null)) // expect: false
 
 // 8-bit clean.
-IO.print("a\0b\0c".iterate(null)) // expect: 0
-IO.print("a\0b\0c".iterate(0)) // expect: 1
-IO.print("a\0b\0c".iterate(1)) // expect: 2
-IO.print("a\0b\0c".iterate(2)) // expect: 3
-IO.print("a\0b\0c".iterate(3)) // expect: 4
-IO.print("a\0b\0c".iterate(4)) // expect: false
+System.print("a\0b\0c".iterate(null)) // expect: 0
+System.print("a\0b\0c".iterate(0)) // expect: 1
+System.print("a\0b\0c".iterate(1)) // expect: 2
+System.print("a\0b\0c".iterate(2)) // expect: 3
+System.print("a\0b\0c".iterate(3)) // expect: 4
+System.print("a\0b\0c".iterate(4)) // expect: false
 
 // Iterates over invalid UTF-8 one byte at a time.
-IO.print("\xef\xf7".iterate(null)) // expect: 0
-IO.print("\xef\xf7".iterate(0)) // expect: 1
-IO.print("\xef\xf7".iterate(1)) // expect: false
+System.print("\xef\xf7".iterate(null)) // expect: 0
+System.print("\xef\xf7".iterate(0)) // expect: 1
+System.print("\xef\xf7".iterate(1)) // expect: false