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/add.wren

@@ -1,10 +1,10 @@
 var a = []
 a.add(1)
-IO.write(a) // expect: [1]
+IO.print(a) // expect: [1]
 a.add(2)
-IO.write(a) // expect: [1, 2]
+IO.print(a) // expect: [1, 2]
 a.add(3)
-IO.write(a) // expect: [1, 2, 3]
+IO.print(a) // expect: [1, 2, 3]
 
 // Returns added element.
-IO.write(a.add(4)) // expect: 4
+IO.print(a.add(4)) // expect: 4