pcomb: parser combinators for the masses

This is a tiny parser combinator library for rust. Combinators allow the ability to easily compose several parsing functions to produce a much larger parser with easy control over output types and control flow.

At the moment, this library only supports parsing from string slices, and requires that one of the output types supports std::iter::Extend. Crucially, it only allows statically generating combinators via the use of rust's expressive generics. Dynamically composing combinators is in the works, but currently, trait objects do not function.