Add fuzz test coverage

"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](https://github.com/rust-fuzz/cargo-fuzz).