"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/string/unicode_escapes.wren

@@ -1,17 +1,17 @@
 // One byte UTF-8 Sequences.
-IO.print("\u0041")     // expect: A
-IO.print("\u007e")     // expect: ~
+System.print("\u0041")     // expect: A
+System.print("\u007e")     // expect: ~
 
 // Two byte sequences.
-IO.print("\u00b6")     // expect: ¶
-IO.print("\u00de")     // expect: Þ
+System.print("\u00b6")     // expect: ¶
+System.print("\u00de")     // expect: Þ
 
 // Three byte sequences.
-IO.print("\u0950")     // expect: ॐ
-IO.print("\u0b83")     // expect: ஃ
+System.print("\u0950")     // expect: ॐ
+System.print("\u0b83")     // expect: ஃ
 
 // Capitalized hex.
-IO.print("\u00B6")     // expect: ¶
-IO.print("\u00DE")     // expect: Þ
+System.print("\u00B6")     // expect: ¶
+System.print("\u00DE")     // expect: Þ
 
 // TODO: Syntax for Unicode escapes > 0xffff?