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/divide.wren

@@ -1,5 +1,5 @@
-IO.write(8 / 2)        // expect: 4
-IO.write(12.34 / -0.4) // expect: -30.85
+IO.print(8 / 2)        // expect: 4
+IO.print(12.34 / -0.4) // expect: -30.85
 
 // TODO: Unsupported RHS types.
 // TODO: Divide by zero.