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_byte_sequence/subscript.wren

@@ -0,0 +1,24 @@
+// Bytes:
+//        012345678
+// Chars: sø mé ஃ
+var bytes = "søméஃ".bytes
+
+IO.print(bytes[0]) // expect: 115
+IO.print(bytes[1]) // expect: 195
+IO.print(bytes[2]) // expect: 184
+IO.print(bytes[3]) // expect: 109
+IO.print(bytes[4]) // expect: 195
+IO.print(bytes[5]) // expect: 169
+IO.print(bytes[6]) // expect: 224
+IO.print(bytes[7]) // expect: 174
+IO.print(bytes[8]) // expect: 131
+
+IO.print(bytes[-9]) // expect: 115
+IO.print(bytes[-8]) // expect: 195
+IO.print(bytes[-7]) // expect: 184
+IO.print(bytes[-6]) // expect: 109
+IO.print(bytes[-5]) // expect: 195
+IO.print(bytes[-4]) // expect: 169
+IO.print(bytes[-3]) // expect: 224
+IO.print(bytes[-2]) // expect: 174
+IO.print(bytes[-1]) // expect: 131