`HashMap.byKey`, `HashMap.byValue` and `HashMap.byPair` should be callable with a const object

[a-ludi]

Hi, the iterator methods of HashMap should be callable with a const object. As far as I can tell, it would suffice to add const keyword to the signature.

[rr0gi]

🙏

[rr0gi]

afaics simply adding const doesn't work, I figure it is because of refcounted :

        // Buckets form contigous array. We keep this array refcounted, so that
        // we can store reference to it from byPair, byKey, ... even if hashtable itself cleared or
        // destroyed.

but const versions would be appreciated nonetheless