Allow to hash extensions, thus allowing them as map keys. - #25

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My only request is to implement the mixing via hashing a tuple rather than addition, as recommended here https://docs.python.org/3/reference/datamodel.html#object.__hash__.. That way we can leverage whatever Python thinks the best way is to mix component hashes.

Good point; pull request updated.

Thanks, squashed and cherry-picked to 4c461ed in devel branch. It will be in the next release soon.