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/setter/same_name_as_method.wren

@@ -1,6 +1,6 @@
 class Foo {
-  bar = value { IO.write("set") }
-  bar { IO.write("get") }
+  bar = value { IO.print("set") }
+  bar { IO.print("get") }
 }
 
 var foo = new Foo