Adding multi-key commitments to LNPBP-1 from LNPBP-2 about lnpbps HOT 1 CLOSED

Comments from the discussion

S> Yes, it seems pretty logical to group these two procedures together, since one is a special case of the other. As a side note, I would like to clarify whether the user who commits to one/many private keys can somehow “check” at a later date how many keys he has used? Will this information not be lost if the procedures are merged?

M> It is seen from the bitcoin script, which is shipped (before the output is spent) as a part of the client-validated state.

F> There is a point that is not clear to me in lnpbp-1. In the second point of the specification paragraph it states:
"Compute HMAC-SHA256 of the lnbp1_msg and P, named tweaking factor: f = HMAC_SHA256(s, P)"
However s is not previously defined, so I guess it refers to the message string of lnbp1_msg. If so for clarity in this sentence we could put directly lnbp1_msg instead of s.

Moreover this could also cause extra confusion in the context of lnpbp-2 where S is used to indicate the sum of all public keys in a script

Related issues and PRs:

#40
#62
LNP-BP/rust-lnpbp#120

from lnpbps.