TINKERPOP-3232 Coerce set into list when it contains non-hashable elements

[xiazcy]

https://issues.apache.org/jira/browse/TINKERPOP-3232

Originally thought of using HashableDict, but ended up with deserializing into list instead due to potential hash collision and silent mutations of nested objects. Given gremlin-go also uses lists to back its custom set type, this wouldn't be an exclusion. Also this best preserves the inner data and type, which is usually what users operate on.

However, I did keep a commit with the HashableDict.of() in case we favor that solution instead. Note there could be an improvement to this, where we use frozenset for set instead of tuple in the HashableDict implementation.

(I will add CHANGELOG entry after the decision is made on the approach.)

CHANGELOG added in favor of deserializing into list to minimize changes to inner results.

VOTE +1

[Cole-Greer]

VOTE +1

[GumpacG]

Could you add to the docs for limitation

tinkerpop/docs/src/reference/gremlin-variants.asciidoc

Line 2879 in 69e2f02

* Traversals that return a `Set` *might* be coerced to a `List` in Python. In the case of Python, number equality

?
Seems like it's already a known limitation, would be nice to specify for non-hashable elements as well.

[kenhuuu]

VOTE +1