findall-variation to better support setof/3

[UWN]

The built-in findall/3 is used to implement setof/3 incurring a lot of unnecessary overheads

1. Duplicate leaf answers are not detected
2. The precise order of answers is not required, they will be sorted anyway
3. Identical ground subterms between leaf answers are not shared
4. Non-ground subterms between leaf answers that are variants of each other and that will become identical directly after findall/3 are not shared

Hashing may now be possible (1 and 2).

Ideally, also hashing of all identical ground subterms (3).

All of this would be for the benefit of setof/3 and (to a lesser degree) of bagof/3. (#3211)

[dougransom]

I have been thinking about findall lately on another tangent #3228