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/list/subscript.wren

@@ -1,12 +1,12 @@
 // Returns elements.
 var list = ["a", "b", "c", "d"]
-IO.write(list[0]) // expect: a
-IO.write(list[1]) // expect: b
-IO.write(list[2]) // expect: c
-IO.write(list[3]) // expect: d
+IO.print(list[0]) // expect: a
+IO.print(list[1]) // expect: b
+IO.print(list[2]) // expect: c
+IO.print(list[3]) // expect: d
 
 // Allows indexing backwards from the end.
-IO.write(list[-4]) // expect: a
-IO.write(list[-3]) // expect: b
-IO.write(list[-2]) // expect: c
-IO.write(list[-1]) // expect: d
+IO.print(list[-4]) // expect: a
+IO.print(list[-3]) // expect: b
+IO.print(list[-2]) // expect: c
+IO.print(list[-1]) // expect: d