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/assignment/associativity.wren

@@ -4,6 +4,6 @@ var c = "c"
 
 // Assignment is right-associative.
 a = b = c
-IO.write(a) // expect: c
-IO.write(b) // expect: c
-IO.write(c) // expect: c
+IO.print(a) // expect: c
+IO.print(b) // expect: c
+IO.print(c) // expect: c