More stuff for working with strings and bytes!

- "\x" escape sequence to put byte values in strings: "\x34"
- String.byteAt(index) gets value of byte in string.
- String.bytes returns a raw sequence of bytes for a string.
- String.codePointAt(index) gets the code point at an offset as a raw number.

builtin/core.wren

@@ -92,7 +92,19 @@ class Sequence {
   }
 }
 
-class String is Sequence {}
+class String is Sequence {
+  bytes { new StringByteSequence(this) }
+}
+
+class StringByteSequence is Sequence {
+  new(string) {
+    _string = string
+  }
+
+  [index] { _string.byteAt(index) }
+  iterate(iterator) { _string.iterateByte_(iterator) }
+  iteratorValue(iterator) { _string.byteAt(iterator) }
+}
 
 class List is Sequence {
   addAll(other) {