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/io/write.wren

@@ -3,10 +3,10 @@ class Foo {
 }
 
 // Calls toString on argument.
-IO.write(new Foo) // expect: Foo.toString
+IO.print(new Foo) // expect: Foo.toString
 
 // Returns argument.
-var result = IO.write(123) // expect: 123
-IO.write(result == 123) // expect: true
+var result = IO.print(123) // expect: 123
+IO.print(result == 123) // expect: true
 
 // TODO: What if toString on argument doesn't return a string?