Make`headValidatorHash` a parameter to the `v_commit` validator

[abailly-iohk]

This can only be done when v_head is not parameterized by the initial/commit Address anymore.

We would exchange finding inputs in collectCom/abort by address with finding them by looking at values (PTs).

It is not clear whether this will help us in any case, it appears to be "just cleaner", but that is a weak argument of doing this.

[abailly-iohk]

Seems like we cannot actually implement this.

[ch1bo]

Since #385 we can implement this now.. if we want :)