Upgrade Rust bdk to alpha 3

[thunderbiscuit]

This PR updates the version of bdk and associated libraries to the alpha 3 version.

The majority of the changes come from the introduction of new error enums. At the moment they are all erased into this one Alpha3Error::Generic error type. Further commits will pull the required errors one at a time and implement them until the generic variant is not required anymore.

[reez]

This looks great other than 1 requested change-

I'd like to not include the dependency thiserror since I think we can do without it, and then use something like this in error.rs instead:

#[derive(Debug)]
pub enum Alpha3Error {
    Generic,
}

impl fmt::Display for Alpha3Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Alpha3Error::Generic => write!(f, "Error in FFI"),
        }
    }
}

impl std::error::Error for Alpha3Error {}

That matches for Alpha3Error what I've got for CalculateFeeError (which matching my code isn't really the important thing, but not including the dependency thiserror is more what I'm going for). With those changes thiserror can be removed and things compile on my end and cargo test --lib still passes locally. But let me know if you have any thoughts related to that or if I'm missing anything.

Other than that this looks great!

[thunderbiscuit]

Oh yeah great idea!

[reez]

LGTM 🚀

ACK 54beb23