Add satisfyMaybe

I find odd that attoparsec has
satisfyWith :: (Word8 -> a) -> (a -> Bool) -> Parser a
but not
satisfyMaybe :: (Word8 -> Maybe a) -> Parser a
since that is what I want most of the time.

Also, you can simulate satisfyWith using satisfyMaybe but not the other way round.

Start by locating satisfyWith in the attoparsec parser API and review the surrounding combinators. Use the requested signature and behavior as the acceptance criteria: a Word8-to-Maybe function should yield a Parser that accepts Just values and rejects Nothing.

Written by the indexing model from the issue text.