Tidy up ellswift before the next major version release

[shinghim]

In #752, Party was introduced to replace ElligatorSwiftParty. As part of the changes, shared_secret and shared_secret_with_hasher were updated to take Into<Party> to preserve backwards compatibility. These functions should probably just be taking Party instead of Into<Party> after ElligatorSwiftParty is removed in the next major version release.

[apoelstra]

We won't remove ElligatorSwiftParty in the next release. I'd give it at least two releases, maybe more.

But will keep this issue open to track it.