ToEncodableKey impls

tor-keymgr/src/keystore.rs has impls of ToEncodableKey for a whole bunch of key types, generally in terms of underlying actual key types.

1. IMO this is the wrong place. The place that defines a newtype for a key role should should provide the ToEncodableKey implementation. I think fixing this might involve separating tor-keymgr into two crates, one of which can exist below other crates. Alternatively, all of tor-keymgr could be moved to below tor-llcrypto, but that doesn't seem right.

2. The impls for these newtypes are very formulaic. They should be derived or provided via a blanket impl or helper trait impl or something.