Types from mintlayer-core-primitives should probably not be used as part of the protocol

[ImplOfAnImpl]

Currently, the app's protocol (i.e. the messages exchanged between the app and the host) mostly consists of the types from mintlayer-core-primitives, which is not great:

• This feels somewhat wrong conceptually. Also, the core-primitives' types mention things (such as orders V0) that the ledger app ideally shouldn't know about.
• More importantly, in order to be able to detect change outputs (see Change outputs should be omitted from review #17), the output destinations should be representable via a derivation path. Which means that we can't use the normal Destination type in tx outputs. In Fixing issues #23 change output detection was implemented by adding change_path to TxOutputData, which is then validated against the actual destination.

See the TODO at the top of crates/messages/src/lib.rs.