Make `union_Some` a ``, add `union_None` and `union_is_Some`.

Merged Robbert Krebbers requested to merge robbert/option_union into master

This makes it more consistent with other Some lemmas, that are also bidirectional.

See also the discussion !466 (comment 91517)

The change to union_Some is not backwards compatible, so we need to either provide a new lemma (with a new name, but what?) for the direction, or we need to update existing code. It's not clear to me how much union_Some is used in reverse dependencies.

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