High Level OpenPGP v6 Key Generation

[vanitasvitae]

Hey! In this PR, I'm working on a high-level API for OpenPGP key generation (v6 only for now). Let me know, what you think of the design sketched out in OpenPGPV6KeyGenerator :)

I tried to keep the API straight-forward, but still allow modifications of the (hashed) signature subpacket areas by the use of callbacks which the user can hook into, to modify the signature subpackets prior to signature generation.

The PR also adds key generator classes for individual keys, such that the risk of making mistakes during key generation (e.g. using wrong/weak parameters) is minimized.

I used PGPainless as orientation for the API design, but kept the generation API a bit slimmer for now. Let me know what you think of the direction the API is heading.