Resolving TODOs for `from_unique_ptr` `void_cast_raw_ptr`. Adding test to...

Resolving TODOs for `from_unique_ptr` `void_cast_raw_ptr`. Adding test to exercise the path through `cast`. The path through init.h is missing a test that would fail if the flag is incorrect. The plan is to systematically cover all situations (there are many that are not exercised for shared_from_this).