Draft: use Ed25519 instead of RSA by default, drop openssl dep

[ljedrz]

While I was looking at the upcoming go-ipfs 0.7 changes, the one that stood out to me most was the switch from RSA to Ed25519 keys by default; since the only reason why we're shipping openssl is that we be default generate RSA keys, this seemed to me like a good opportunity to ditch it and free ourselves from one of the trickiest dependencies.

Since I might have been a bit "heavy-handed" in the way I handled these changes and since they are quite severe, I'm posting it as a draft so that we can discuss this further, see how the CI changes fare etc. There's some work left wrt updating the README etc., but "the meat" is already here.

This would obviate https://github.com/rs-ipfs/rust-ipfs/pull/128. Cc https://github.com/rs-ipfs/rust-ipfs/issues/170, https://github.com/rs-ipfs/rust-ipfs/issues/96

[ljedrz]

Nice, the CI seems to agree with this approach; I'll just force-push the removal of libssl-dev I forgot to include initially.

[Logarithmus]

More than a year passed... Please merge this finally