Add overrides for the iterator methods that do not get the key

[Rüdiger Klaehn]

Materializing the keys out of the compact sequences is a significant part of the work. But in some cases you only need the value.

There should be variants of the 2 iter methods that return (Offset, Value) instead of (Offset, Key, Value). That would be faster and fewer mallocs.

[Rüdiger Klaehn]

Thinking further about this, you might need a custom projection of the CompactSeq for the keys. E.g. you might need the lamports for sorting, but not the tags.

So maybe extend the iterator with a projection function that is given a reference to the KeySeq and a reference to the bitmap and is supposed to produce an iterator of results?