`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.
