"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/comments/block.wren

@@ -1,5 +1,5 @@
 // In middle of line.
-IO.print/* ... */(/* */"ok"/* */) // expect: ok
+System.print/* ... */(/* */"ok"/* */) // expect: ok
 
 // Nested.
-IO.print(/* in /* nest */ out */"ok") // expect: ok
+System.print(/* in /* nest */ out */"ok") // expect: ok

test/language/comments/block_at_eof.wren

@@ -1,2 +1,2 @@
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok
 /* comment */

test/language/comments/line_at_eof.wren

@@ -1,2 +1,2 @@
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok
 // comment

test/language/comments/unicode.wren

@@ -8,4 +8,4 @@
 
 // TODO: What about combining characters?
 
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok

test/language/comments/unterminated_block.wren

@@ -1,3 +1,3 @@
 // expect error line 3
-IO.print("nope") /*
+System.print("nope") /*
 oops

test/language/comments/unterminated_nested_block.wren

@@ -1,4 +1,4 @@
 // expect error line 4
-IO.print("nope") /* /* /*
+System.print("nope") /* /* /*
 */
 oops