"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/starts_with.wren

@@ -1,14 +1,14 @@
-IO.print("abcd".startsWith("cd")) // expect: false
-IO.print("abcd".startsWith("a")) // expect: true
-IO.print("abcd".startsWith("abcd")) // expect: true
-IO.print("abcd".startsWith("abcde")) // expect: false
-IO.print("abcd".startsWith("")) // expect: true
+System.print("abcd".startsWith("cd")) // expect: false
+System.print("abcd".startsWith("a")) // expect: true
+System.print("abcd".startsWith("abcd")) // expect: true
+System.print("abcd".startsWith("abcde")) // expect: false
+System.print("abcd".startsWith("")) // expect: true
 
 // Non-ASCII.
-IO.print("søméthîng".startsWith("sømé"))  // expect: true
-IO.print("søméthîng".startsWith("some")) // expect: false
+System.print("søméthîng".startsWith("sømé"))  // expect: true
+System.print("søméthîng".startsWith("some")) // expect: false
 
 // 8-bit clean.
-IO.print("a\0b\0c".startsWith("a"))  // expect: true
-IO.print("a\0b\0c".startsWith("a\0"))  // expect: true
-IO.print("a\0b\0c".startsWith("b\0"))  // expect: false
+System.print("a\0b\0c".startsWith("a"))  // expect: true
+System.print("a\0b\0c".startsWith("a\0"))  // expect: true
+System.print("a\0b\0c".startsWith("b\0"))  // expect: false