Change the repository structure to contain multiple packages

[evgeny-osipenko]

Resolves the issue #43.

[jprupp]

Hello Evegny,

I haven't looked at the code yet, but I would like to understand your goals with these changes. I can think of two possibilities:

• Expose internal types and functions from this library in order to ease development of third-party packages providing additional features.
• Allow extensions to this library to exist as separate packages, maintained by their own developers but hosted in this repository as their canonical locations.

With this information, I will set to review the request.

Greetings

[evgeny-osipenko]

Yes, indeed, that's exactly the goals for this PR.

And #45 is exactly one of such extensions, which implements the recoverable signatures.

[evgeny-osipenko]

@jprupp Is there any progress on reviewing this PR?

[jprupp]

I'm very sorry for the delay. The changes look good to me. When I'm back at a proper desktop computer, I'll merge these changes.

[jprupp]

I merged these changes plus a few additions using the command line, but GitHub isn't detecting it correctly. I'll close this pull request manually.