basic_unique_ptr: reset on move assignment

[AdelKS]

Hello!

While I was running asan and tsan tests on my project, I met a memory leak. It took me a while, but I found the culprit ! I think cista definitely needs to have its tests run with the address sanitizer.

Now that I think about it: maybe it's worth specialising std::swap for cista's classes.

Adel

[felixguendling]

The CI tests run already with -fsanitizer=address,undefined. The problem in this case is test coverage. I'm open to merge PRs that increase test coverage (or std::swap specializations).