add getters for rsa key params p, q, n, e and d

[repomaa]

This should be useful for serializing into JWK

[CImrie]

Thanks @jreinert . We are trying to get this integrated into the Crystal stdlib so that's the reason I haven't merged this in yet (if at all). @bararchy was preparing the PR for the stdlib. @bararchy is there any update on this? Wondering how we can merge this in with the stdlib in mind 💪

[stakach]

I think just merge this in. Might as well keep this project running in the mean time.
Also I'm happy to take over maintenance if you want to make me a collaborator (if you don't mind)

You can see some of my companies crystal work over here: https://github.com/spider-gazelle

Side note, this pull doesn't work properly with LibreSSL