typeAssertStatus deals with the 3rd form of type assertion ("_, ok"), for
when one does not care about the result of the assertion itself.
Some cases for it, which are already fixed for the two other forms of
type assertions, had not been fixed for this form yet.
Therefore, this change fixes such cases for this form, while integrating
typeAssertStatus to the same code path as for the two other forms.
The long-form (with comma-ok) ones were already fixed but the short-form
ones were not because they were in a completely different code path.
This PR also refactors the code so that both short-form and long-form
are now merged in the same function.
N.B: even though most (all?) cases seem to now be supported, one of them
still yields a result that does not satisfy reflect's Implements method
yet. It does not prevent the resulting assertion to be usable though.
N.B2: the code path for the third-form (_, ok) hasn't been fixed and/or
refactored yet.
Fixes#919
