Splitting up rcgen's lib.rs into smaller modules

[est31]

Suggested by @cpu in #137.

I don't think we should overdo this, but there is certainly potential to split off modularized components from the main lib.rs file:

• Split off oid module: fbd48cc
• Split off csr module: e137bef
• Split off key_pair module: 59253f5
• Split off sign_algo module: 8b95395
• Split off error module: e39892c
• Split off crl module: c0344db
• Replace OID_FOO with oid::FOO
• (for the next breaking release, so probably after the next minor one) redesign the public API to be a little bit more modularized: move csr and crl stuff into own modules in the public API, maybe also the algos?

[cpu]

It feels like we're in pretty good shape here. I checked off the OID task, it was accomplished in f2789b3

Are there any other tasks folks had in mind for the last bullet or should we consider this done?

[djc]

I think we're in a good place on this!

[est31]

I think we can close this as done. For the last bullet point, if it makes sense we can track it separately.