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/variable/scope_reuse_in_different_blocks.wren

@@ -1,9 +1,9 @@
 {
   var a = "first"
-  IO.write(a) // expect: first
+  IO.print(a) // expect: first
 }
 
 {
   var a = "second"
-  IO.write(a) // expect: second
+  IO.print(a) // expect: second
 }