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.

test/core/string/bytes.wren

@@ -0,0 +1,6 @@
+// Bytes:           11111
+//        012345678901234
+// Chars: sø mé ஃ  thî ng
+var s = "søméஃthîng"
+
+IO.print(s.bytes is StringByteSequence) // expect: true