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/field/nested_class.wren

@@ -1,17 +1,17 @@
 class Outer {
   method {
     _field = "outer"
-    IO.write(_field) // expect: outer
+    IO.print(_field) // expect: outer
 
     class Inner {
       method {
         _field = "inner"
-        IO.write(_field) // expect: inner
+        IO.print(_field) // expect: inner
       }
     }
 
     (new Inner).method
-    IO.write(_field) // expect: outer
+    IO.print(_field) // expect: outer
   }
 }