[keys] Add a way to restrict the networks in which keys are valid

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.

src/descriptor/error.rs

@@ -31,7 +31,9 @@ pub enum Error {
     InvalidPrefix(Vec<u8>),
     HardenedDerivationOnXpub,
     MalformedInput,
+
     KeyParsingError(String),
+    Key(crate::keys::KeyError),
 
     Policy(crate::descriptor::policy::PolicyError),
 
@@ -50,6 +52,16 @@ pub enum Error {
     Hex(bitcoin::hashes::hex::Error),
 }
 
+impl From<crate::keys::KeyError> for Error {
+    fn from(key_error: crate::keys::KeyError) -> Error {
+        match key_error {
+            crate::keys::KeyError::Miniscript(inner) => Error::Miniscript(inner),
+            crate::keys::KeyError::BIP32(inner) => Error::BIP32(inner),
+            e @ _ => Error::Key(e),
+        }
+    }
+}
+
 impl std::fmt::Display for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{:?}", self)