Delimited parsers

A common situation is to parse an encoding prefixed by its length. So you first parse the length as an integer `n`, and then you would like to run a (sub)parser `p :: Parser a` only on the next `n` bytes. I could think of two solutions for users today:

- Use `take` to get the `ByteString` and apply `parseOnly p`. However, we lose source position information in case the subparser fails, and we have to keep the whole `ByteString` in memory.

- Wrap `Parser` (e.g., with a few monad transformers) to track things like the number of bytes read; that would allow combinators like the ones I have in mind. However, this is rather heavyweight to implement. Does an existing library already offer this? I also suspect this approach would have more overhead than necessary.

It would be nice for attoparsec to have combinators to delimit the input that a subparser gets to see, like `span` and `splitAt` in [pipes-parse](http://hackage.haskell.org/package/pipes-parse-3.0.8/docs/Pipes-Parse.html).

What do you think of such an addition? Is there a better solution?