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/remove_at.wren

@@ -1,27 +1,27 @@
 var a = [1, 2, 3]
 a.removeAt(0)
-IO.write(a) // expect: [2, 3]
+IO.print(a) // expect: [2, 3]
 
 var b = [1, 2, 3]
 b.removeAt(1)
-IO.write(b) // expect: [1, 3]
+IO.print(b) // expect: [1, 3]
 
 var c = [1, 2, 3]
 c.removeAt(2)
-IO.write(c) // expect: [1, 2]
+IO.print(c) // expect: [1, 2]
 
 // Index backwards from end.
 var d = [1, 2, 3]
 d.removeAt(-3)
-IO.write(d) // expect: [2, 3]
+IO.print(d) // expect: [2, 3]
 
 var e = [1, 2, 3]
 e.removeAt(-2)
-IO.write(e) // expect: [1, 3]
+IO.print(e) // expect: [1, 3]
 
 var f = [1, 2, 3]
 f.removeAt(-1)
-IO.write(f) // expect: [1, 2]
+IO.print(f) // expect: [1, 2]
 
 // Return the removed value.
-IO.write([3, 4, 5].removeAt(1)) // expect: 4
+IO.print([3, 4, 5].removeAt(1)) // expect: 4