`Wallet` stores the descriptors' checksum in the database for safety.
Previously, the checksum used was a checksum of a descriptor that
already had a checksum.
This PR allows for backward-compatibility of databases created with this
bug.
If `exclude_hash` is set, we split the input data, and if a checksum
already existed within the original data, we check the calculated
checksum against the original checksum.
Additionally, the implementation of `IntoWalletDescriptor` for `&str`
has been refactored for clarity.
`get_checksum_bytes` returns a descriptor checksum as `[u8; 8]` instead
of `String`, potentially improving performance and memory usage.
In addition to this, since descriptors only use charaters that fit
within a UTF-8 8-bit code unit, there is no need to use the `char` type
(which is 4 bytes). This can also potentially bring in some performance
and memory-usage benefits.
Change the return type of the `descriptor!()` macro and `ToWalletDescriptor` to
avoid having to map errors.
Also introduce more checks to validate descriptors built using the macro.
