A new `ExtendedKey` type has been added, which is basically an enum of
`bip32::ExtendedPubKey` and `bip32::ExtendedPrivKey`, with some extra metadata
regarding the `ScriptContext`.
This type has some methods that make it very easy to extract its content as
either an `xprv` or `xpub`.
The `DerivableKey` trait has been updated so that the user now only has to
implement a method (`DerivableKey::into_extended_key()`) to perform the
conversion into an `ExtendedKey`.
The method that was previously called `add_metadata()` has now been renamed
to `into_descriptor_key()`, and it has
a blanket implementation.
Thanks to the `ToWalletDescriptor` trait we can also very easily validate the checksum
for descriptors that are loaded from strings, if they contain one. Fixes#20.
