Implement Lightning message signing

[sr-gi]

Both lnd and c-lightning do implement a "Lightning Message Signing" protocol to allow nodes to sign/verify messages from other nodes.

The signature format is as described in https://twitter.com/rusty_twit/status/1182102005914800128

zbase32(SigRec(SHA256(SHA256("Lightning Signed Message:" + msg))))
zbase32 from https://philzimmermann.com/docs/human-oriented-base-32-encoding.txt 
SigRec has first byte 31 + recovery id, followed by 64 byte sig. 

It may make sense to add zbase32 to LDK so the signature algo can be implemented, or even implement it within LDK and add bindings.

[sr-gi]

I implemented this for The Eye of Satoshi a while back, so I can take care to port it to Rust if you think it makes sense to have it here.

[TheBlueMatt]

Sure, if you feel so inclined, go for it!