Fix `vec_is()` by comparing ptypes

[lionel-]

We could make vec_is() generic by requiring the result of vec_ptype() to only contain essential attributes. Methods for vec_ptype() would need to remove any details attributes. Ideally they'd just return a static ptype. vec_is() would then compare the ptypes with identical().

vec_ptype() is already generic, although it is currently documented as a only a performance generic. Here we are considering giving it a specific role in the genericity API of vctrs.