core: Change owns to take a StealthAddress

[moCello]

These changes are required after the deletion of the Ownable trait, in order for the functions to take both Notes and Licenses

[xevisalle]

it looks good to me, but I have two questions:

• since the code with note still works, why the change is needed?
• if the change is needed, maybe sk.owns(sa) feels weird? sk.check_sa(sa) or similar is more appropiate?

[HDauven]

• if the change is needed, maybe sk.owns(sa) feels weird? sk.check_sa(sa) or similar is more appropiate?

I prefer sk.owns(sa) because it clearly conveys the relationship between the secret key and the ownership of the stealth address. It is much more declarative.

[moCello]

it looks good to me, but I have two questions:

* since the code with `note` still works, why the change is needed?

Because it doesn't work with Licenses.

* if the change is needed, maybe `sk.owns(sa)` feels weird? `sk.check_sa(sa)` or similar is more appropiate?

This would create more unneeded changes downstream that I like to avoid at this point.