Use miniscript::DescriptorPublicKey

This allows us to remove all our custom "ExtendedDescriptor" implementation since that is
now built directly in miniscript.

src/lib.rs

@@ -47,7 +47,6 @@ pub mod blockchain;
 pub mod database;
 pub mod descriptor;
 pub mod psbt;
-pub mod signer;
 pub mod types;
 pub mod wallet;