Allow to compare Mutez values

[m-kus]

Hey guys, a small improvement for the Mutez type that enables partial ordering. It is a nice feature to have, e.g. in my case I needed to compare transaction amount + fees against the source account balance.

[RomarQ]

Thanks for the contribution :+1:

We will cherry-pick your commit upstream.

[RomarQ]

Your changes have been merged into the develop branch. https://github.com/airgap-it/tezos-rust-sdk/commit/273f3096601a31f63428265557e32f60fd79185c

[m-kus]

Thanks!