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/super/indirectly_inherited.wren

@@ -1,6 +1,6 @@
 class A {
   foo {
-    IO.write("A.foo")
+    IO.print("A.foo")
   }
 }
 
@@ -8,7 +8,7 @@ class B is A {}
 
 class C is B {
   foo {
-    IO.write("C.foo")
+    IO.print("C.foo")
     super.foo
   }
 }