Let chunks be strings

[travisbrown]

I'm working on a new iteratee-parsing module with iteratee-powered parser combinators, and I really want to be able to parse strings as streams of characters without representing chunks as sequences of characters. This PR is a first step in that direction.

There's no measurable impact on performance (at least in the benchmarks I've looked at so far), since the IsSeqLike instances get instantiated once for each type involved, and in the case where the chunks are sequences, the conversion is just identity.

This is a breaking change, but only to fairly internal APIs.

[travisbrown]

Oh, damn, I forgot IsSeqLike is new in 2.11. That's what I get for not running all tests locally. I'll figure out a workaround this evening.