`hash_and_equals` support for extension types

[pq]

[eernstg]

(No-op? An extension type declaration cannot have members named == or hashCode.)

[pq]

@eernstg: exactly right. I just want to make sure we don't overreport.

[eernstg]

Ah, of course, I didn't think about that. Sorry about the noise. ;-)

[pq]

Not noise at all. Any feedback is appreciated! (And to be fair, I could have added some context to the initial issue.)