feat: simplify stacks transaction signing

[djordon]

Description

Closes #262. What is left is documentation on what the flow is supposed to look like, which is what #267 is for.

This PR simplifies part of the process of signing stacks transactions. The coordinator can use the code here to generate transactions that other signers can sign and combine the signatures into a final transaction. The final transaction that will (very likely) pass verification on a stacks-core node.

Changes

1. Add functions for generating the multi-sig stacks address of the all signers participating in a PoX cycle.
2. Add a trait for specifying what is necessary to create a stacks contract call transaction.
3. Add traits and functions for signing a transaction.

Testing information

There are unit tests that check that the final signed StacksTransaction will pass verification. The integration tests that check validation on a stacks node will be done in #263 and #264.

[netrome]

Really nice stuff! My main consideration is how we represent the public keys in the wallet type, where I think it should be a BTreeMap of the stacks variant of the public key type in this case.

[AshtonStephens]

Looks good to me!