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/global.wren

@@ -1,8 +1,8 @@
 var a = "before"
-IO.write(a) // expect: before
+IO.print(a) // expect: before
 
 a = "after"
-IO.write(a) // expect: after
+IO.print(a) // expect: after
 
-IO.write(a = "arg") // expect: arg
-IO.write(a) // expect: arg
+IO.print(a = "arg") // expect: arg
+IO.print(a) // expect: arg