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/closure/use_global_in_method.wren

@@ -3,11 +3,11 @@ var global = "global"
 
 class Foo {
   method {
-    IO.write(global)
+    IO.print(global)
   }
 
   static classMethod {
-    IO.write(global)
+    IO.print(global)
   }
 }