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/string/literals.wren

@@ -1,2 +1,2 @@
-IO.write("".count)   // expect: 0
-IO.write("a string") // expect: a string
+IO.print("".count)   // expect: 0
+IO.print("a string") // expect: a string