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/language/string/byte_escapes.wren

@@ -0,0 +1,12 @@
+var s = "\x00\x12\x34\x56\x78\xab\xCD\xfFf"
+
+IO.print(s.byteAt(0)) // expect: 0
+IO.print(s.byteAt(1)) // expect: 18
+IO.print(s.byteAt(2)) // expect: 52
+IO.print(s.byteAt(3)) // expect: 86
+IO.print(s.byteAt(4)) // expect: 120
+IO.print(s.byteAt(5)) // expect: 171
+IO.print(s.byteAt(6)) // expect: 205
+IO.print(s.byteAt(7)) // expect: 255
+// "f".
+IO.print(s.byteAt(8)) // expect: 102

test/language/string/incomplete_byte_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\x0"

test/language/string/incomplete_byte_escape_at_eof.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\x0

test/language/string/invalid_byte_escape.wren

@@ -0,0 +1,2 @@
+// expect error line 2
+"\x0!"