Add a parser-combinator crate

Parser-combinators are one of the simpler tools for building ad-hoc
parsers. They're a good fit because they are...

* Small: each parser / parser-combinator is around 10 LOC.
* Functional: helix_core strives to be a functional set of utilities
  usable throughout the rest of the editor.
* Flexible: use them to build any sort of ad-hoc parser. In the child
  commit, we'll parse LSP Snippet syntax using these new parser
  combinators.

Why not use an existing parser-combinator crate? Existing popular
parser-combinator crates have histories of making breaking changes
(for example nom and combine).

> Implementation note: I tried to not introduce a new trait since the
> types can be expressed in terms of `impl Fn`s. The trait is necessary
> to build `seq` implementations without a proc macro though, and also
> allows us to use `&'static str`s very conveniently: see the trait
> implementation for `&'static str`.

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