Add functions to create and verify a multisignature bulletproof. Callers must go
through multiple steps to create the full bulletproof

Each party must complete each step in the protocol, and the initiator
finalizes the multisignature bulletproof

Adds serde mod for an optional public key.

Useful for multisig output and atomic swap transaction flow

Add option to create an adaptor signature by signing with an extra
nonce

Build a multisig output with a blank rangeproof, to build the rangeproof
over a number of rounds

Output commit is a sum of a partial commit to the output value and a
commit to zero

Add a function to deserialize a keychain `Identifier` from a BIP32
string

Change rust-secp256k1-zkp dependency to include changes necessary for
atomic swaps

Points to https://github.com/geneferneau/rust-secp256k1-zkp#atomic

**DO NOT MERGE**

Drop this commit after merging rust-secp256k1-zkp changes upstream

Add functions to build an input from a multisig output

Uses the provided shared commitment to build the transaction input

Add functionality for determining if an output is multisig