It only needs std and bincode v2.0.0. We can make bincode an optional dependency as I think implementing bincode traits directly would be a nice addition anyway.

The only thing I can see that we need to do is get rid of anyhow from bdk_file_persist (just return an std::io::Error) and to actually implement the bincode traits on everything. Some things might have to be done without derive since we might want to use consensus encode/decode for various bitcoin types.