Clean up text handling a bit:

- Rename IO.write -> IO.print.
- Make IO.write not print a newline.
- Support \u Unicode escapes in strings.

test/field/use_before_set.wren

@@ -1,5 +1,5 @@
 class Foo {
-  write { IO.write(_field) } // Compile a use of the field...
+  write { IO.print(_field) } // Compile a use of the field...
   init { _field = "value" }  // ...before an assignment to it.
 }