add parseWithLength

[raehik]

Returns parse result along with number of bytes consumed. Maybe useful for low level binary parsing.

• [ ] Add Stateful version
• [ ] Figure out a better name?
• [ ] Add some basic tests (parse a bytestring of length N (QuickCheck prop), parse a Word32)

[raehik]

I found myself wanting this in a library where I define arbitrary null-padded data. I parse a value of the type, then parse(/skip) nulls until a given size is reached. For that, I need to check how many bytes the parser consumed. (Previously I was doing something silly, now I'm taking a second look I realize I should be using such a combinator.)