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/comments/unterminated_nested_block.wren

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