Add support for Collection[X] | Y (for some collection types)

[jhominal]

fixes #743

As discussed in the issue, here is the PR.

Compared to my last comment on the issue:

• I have decided to also include support for non-sequence types deque, set and frozenset. In case you would prefer not supporting these types, I can remove them (I am not sure whether the added support is "worth it" or not.)
• I have removed any attempt to handle Union passthrough does not work when target union uses a new-style type alias as element for passthrough #742, and I expect to apply the fix for Union passthrough does not work when target union uses a new-style type alias as element for passthrough #742 to this PR when it has been designed.
• I have refactored some aspects (parameter names, avoiding parent function scope capture) so that the functions look more similar to the union pass-through strategy.

I must admit, I was not too inspired for tests, there are probably more tests that could be added. The PR is also missing documentation.

[Tinche]

Just letting you know this is on my radar, I'm feeling under the weather this week so it might have to wait a little

[jhominal]

Thank you for commenting about your status!

Do not worry about it - there is really nothing on the line here except a possible improvement to users of cattrs.