Move types to modules, generate docs with rebar3_ex_doc

[rkallos]

This PR started out as an experiment to switch from using edown to using rebar3_ex_doc for generating documentation.

The resulting documentation showed that each module that directly or transitively included shackle.hrl had its own copy of the type declarations in that header file. To clean that up, I moved the type declarations to the modules that seemed the most appropriate. Another option might be to export all types from the shackle module. I'd be happy to make those changes if that's what you'd prefer.

You can generate the docs by running rebar3 ex_doc, running a HTTP server (I like python3 -m http.server) and navigating into /doc in a web browser.

If these instructions are to be believed, publishing docs to Hexdocs is as straightforward as running rebar3 hex publish docs after successfully running rebar3 hex publish.

[lpgauth]

@rkallos can you rebase off master? thanks!

[rkallos]

Done!