Use standardized hash.CloneHash interface

[qmuntal]

Upstream will most likely define a standardized hash clone interface: golang/go#69521 (comment)

type CloneHash interface {
    hash.Hash
    Clone() hash.Hash
}

We should follow suite so that our hash types are compatible with upstream. The only difference with our current approach is that the new Clone method doesn't return an error, which makes sense given that other hash methods are not allowed to return an error. It is also unlikely that OpenSSL will return an error when duplicating a hash unless there is a programmer error or the there is no memory available in the system, in which case it makes sense to panic anyway.

Note that the current Clone methods are not yet included in a release, so this is not a breaking change.