"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/whitespace.wren

@@ -1,26 +1,26 @@
 // No indent
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok
 
 // Indent with space
-  IO.print("ok") // expect: ok
+  System.print("ok") // expect: ok
 
 // Indent with tab
-	IO.print("ok") // expect: ok
+	System.print("ok") // expect: ok
 
 // Indent with space then tab
-  	IO.print("ok") // expect: ok
+  	System.print("ok") // expect: ok
 
 // Indent with tab then space
-	  IO.print("ok") // expect: ok
+	  System.print("ok") // expect: ok
 
 // Indent with mixed tab and space
-  	  	IO.print("ok") // expect: ok
+  	  	System.print("ok") // expect: ok
 
 // Space in with code
-IO . print ( "ok" ) // expect: ok
+System . print ( "ok" ) // expect: ok
 
 // Tab in with code
-IO	.	print	(	"ok"	) // expect: ok
+System	.	print	(	"ok"	) // expect: ok
 
 // Tab and space mixed in with code
-IO	 . 	print 	(	 "ok" 	) // expect: ok
+System	 . 	print 	(	 "ok" 	) // expect: ok