Add fuzz test coverage

[notmandatory]

"More fuzz coverage. I only started writing a couple fuzz targets after having reviewed the 15k lines of sources. These targets can be improved, asserting more invariants and exercising more logic. New, more specific, targets may also be introduced. As BDK currently has no fuzz coverage, and its modular architecture makes it adapted to fuzzing, this is probably the highest value thing to implement to further give assurance in the quality of the library."

Audit report contains two example fuzz targets written against Libfuzzer, as integrated by cargo-fuzz.

[oleonardolima]

Although I'm not well-versed in fuzzing, FWIW I guess we could make use of bitcoinfuzz for differential fuzzing at some point in time, which would test against Core, Mako, and Rust Bitcoin, in particular, which one I'd expected no discrepancies and could be a good baseline.

[oleonardolima]

I'll start taking a look at this one, beginning with the initial fuzz target from Antoine's audit.

[notmandatory]

Some useful related work: Writing Fuzz Targets for Wallets: Avoiding Known Issues

[oleonardolima]

As a side note, I discussed with some folks at bitcoin++ floripa and we could also take a look into fuzzamoto.

It's not clear to me how we could use it yet, and/or what features it's missing for our scenarios, but it can be helpful if we'd like to have some sort of deeper fuzzing in bdk_chain with bitcoind backend, and even do some fuzzing/testing against other wallet implementations (eg. Bitcoin Core). It's still needs research and discussion though.