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/bool/not.wren

@@ -1,3 +1,3 @@
-IO.write(!true)    // expect: false
-IO.write(!false)   // expect: true
-IO.write(!!true)   // expect: true
+IO.print(!true)    // expect: false
+IO.print(!false)   // expect: true
+IO.print(!!true)   // expect: true