export combinators for use in other parser combinators

[mwhitworth]

so that we can incorporate these top-level combinators in other grammars

[mwhitworth]

@dbernheisel - would it be possible to make a release including this fix? It would be required for a new version of my library relative_datetime to be published

[dbernheisel]

Heyo, I'm not at my computer to make the change for this week, so the earliest I can do it is probably Friday or this weekend.

[dbernheisel]

I'm ok to merge this, but since they're exported/public I'll also want to document that these are not part of the API and can break upon any release. The combinators are likely to change every release.

[mwhitworth]

I'll add a header comment to that effect, and fix the unverified commit.

[mwhitworth]

Documentation options for defparsec are limited (to achieve this goal), since it generates its own @doc function and there are no options to control @doc for either the wrapper function or the combinator.

[dbernheisel]

The module is already @moduledoc false so this should be good enough