Express the wallet changeset over the FFI layer

[rustaceanrob]

Description

Builds on #754 to create a fully expressive and serializable ChangeSet type. The goal is for this to enable arbitrary persistence backends across the FFI layer. Ultimately that would close #739 and any further database requests that are outside the immediately supported options.

For context: The iterators in the BDK changesets are BTree based to be no_std compatible and have deterministic ordering of duplicate data across all targets. UniFFI exposes only two iterators across the boundary, Vec and HashMap, but I can try to lay out why I think using these is acceptable.

The goal here is to convert between bdk_wallet::ChangeSet and FFI ChangeSet, and the easiest way to go about that is to just make each field convertible. For the LocalChain::ChangeSet, I iterate over the BTreeMap, which will return the key-value pairs in order. That means the vector will be in order over the FFI layer. Everything else coming from BDK could be ordered by key, but is not required in my opinion to implement the persistence. For instance any mapping from DescriptorId to last revealed index is okay, as the DescirptorId do not necessarily have to be in any particular order. Same goes for Txid or Outpoint mapping to some value. While it is nice to be reproducible, these keys are suitable for a HashMap.

Can elaborate on the next call

Notes to the reviewers

In Swift, and key of a HashMap must implement Equitable and Hashable. We can export Eq and Hash from Rust for Objects, but not for Records from what I can tell. As a result, I made a kind-of-gross wrapper called HashableOutPoint that is an Object so it can be used as a key, but it is convertible to the underlying Record if required. Open to ideas on that one.

Also, I cannot for the life of me figure out the best way to go from a Descriptor<DescriptorPublicKey> into DescriptorPublicKey and vise versa without introducing an unwrap. Feels like we can improve that.

Changelog notice

Checklists

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Bugfixes:

• This pull request breaks the existing API
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