Alternative full node implementations as targets

[dergoegge]

Potential full node targets:

• Btcd
• libbitcoin
• swift-bitcoin
• mako
• Floresta

Needs something similar to rust-bitcoin/corepc for each implementation and a Target impl.

[brunoerg]

I think Floresta would be great as well.

[dergoegge]

Added to the list!

[brunoerg]

Another one that might be interesting and should be easy to integrate since it's a fork of Bitcoin Core is elements.