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/clear.wren

@@ -1,7 +1,7 @@
 var a = [1, 2, 3]
 a.clear
-IO.write(a)       // expect: []
-IO.write(a.count) // expect: 0
+IO.print(a)       // expect: []
+IO.print(a.count) // expect: 0
 
 // Returns null.
-IO.write([1, 2].clear) // expect: null
+IO.print([1, 2].clear) // expect: null