Fix argument passing from trampoline methods

[rhaschke]

This adds some more smart_holder tests that mimic some of my (currently failing) application use cases.
Particularly, they consider roundtrip situations where an object is passed around as a reference:

• Python -> C++ -> Python
• C++ -> Python -> C++

The single failing test in test_unique_ptr_cref_consumer_roundtrip and test_unique_ptr_cref_roundtrip aren't important. I added them just for completeness. However, test_unique_ptr_consumer_roundtrip is. For this latter test, I didn't look into the reason yet. Will continue tomorrow...

[rhaschke]

As expected, the unit test was failing because args were passed by-value and not by-reference. However, it wasn't possible to just change the policy to reference or reference_internal.
reference gave errors, because const unique_ptr& only accepts reference_internal.
reference_internal gave errors, because the parent required for keep_alive is null.
Hence, I decided to introduce a new policy to explicitly indicate passing from from Python to C++ via an overridden method. That's the cleanest solution to correctly react to the special needs of this use case.

[rhaschke]

This PR is failing the unit tests, but only with the valgrind config. I have no idea why. The first failing commit is 4b0b8ae.
@YannickJadoul, did you originally introduce the valgrind config? Do you have any idea, why test_gil_scoped.py fails in this situation? Is valgrind too slow?

[rhaschke]

Superseeded by #2916.