Support conversion of multi-Xprivs into multi-Xpubs

[shesek]

Possible when all hardened derivation steps are shared among all paths (or if there are none). Errors otherwise.

[apoelstra]

ACK 7d3e1e8; successfully ran local tests; the stringly-typed errors are OK since this module is full of them

[apoelstra]

Would like @sanket1729 to take a quick look at this before acking to sanity-check it.

In particular this forbits multixpubs where there are hardened derivations after the split. I assume this is a requirement to be a legal multixpub? Because there isn't any obvious reason we should be forced to have this limit.

[shesek]

assume this is a requirement to be a legal multixpub? Because there isn't any obvious reason we should be forced to have this limit.

Correct, this is a limitation imposed by the current DescriptorMultiXKey structure (can only have one concrete pubkey as the base for derivations) and not something inherent. But supporting it would require some pretty invasive changes and it seemed to me that in the typical case hardened derivation steps are usually shared, and that it can be useful even with that limitation.

[shesek]

Added another commit with some formatting changes that also updates the existing DescriptorXKey::to_public(). Let me know if you'd like me to squash or drop it.

[sanket1729]

LGTM

[sanket1729]

utACK ae64ce6

[apoelstra]

ACK ae64ce6; successfully ran local tests; the stringly-typed errors are OK since this module is full of them