Support parsing integers from `&[u8]`

[jwodder]

The "obvious" way to do this would be for the parsing functions to take S: AsRef<[u8]> so that they can be passed both &str and &[u8], but then how should functions that only consume some of the input return the remainder?

• Add a sealed Haystack (or Input?) trait that is implemented by both &str and &[u8] (with or without &?), and make the parsing functions operate on instances of this trait?