RFC: determinisctic mapping between input and output derivation index

[pythcoiner]

note i removed from the code:

TODO: we should add a new rule in order to 'map' an input derivation path to the funded
output derivation path: multipath of the output should always be the multipath of the
input +2. For instance, if a non conjoined input have a derivation path of
m/84'/0'/0'/<0;1>/12 the output path should be m/84'/0'/0'/<2;3>/12, then if this utxo
fund a new coinjoin, it's linked output should have a derivation path of
m/84'/0'/0'/<4;5>/12, etc...
It makes it easy for the signer to verifying one output belong to self w/o trusting an
external information, and make impossible some atack where the attacker can make the
signer sign 2 input for a single matching output.
One drawback to this is that if an output address is broadcasted in a pool and the pool
isn't finalized, used cannot replaced the output address in the next pool...