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/number/multiply.wren

@@ -1,4 +1,4 @@
-IO.write(5 * 3) // expect: 15
-IO.write(12.34 * 0.3) // expect: 3.702
+IO.print(5 * 3) // expect: 15
+IO.print(12.34 * 0.3) // expect: 3.702
 
 // TODO: Unsupported RHS types.