fix: add missing destructor for CoreMPL and final for nested classes

[knst]

There is no actual memory leak because no data in the the CoreMPL

But it resolves clang's warning:

/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/unique_ptr.h:95:2: warning: delete called on non-final 'bls::CoreMPL' that has virtual functions but non-virtual destructor [-Wdelete-non-abstract-non-virtual-dtor]
        delete __ptr;
        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/unique_ptr.h:396:4: note: in instantiation of member function 'std::default_delete<bls::CoreMPL>::operator()' requested here
          get_deleter()(std::move(__ptr));
          ^
main.cpp: note: in instantiation of member function 'std::unique_ptr<bls::CoreMPL>::~unique_ptr' requested here
static const std::unique_ptr<bls::CoreMPL> pSchemeMLP{std::make_unique<bls::BasicSchemeMPL>()};

[github-actions]

'This PR has been flagged as stale due to no activity for over 60
days. It will not be automatically closed, but it has been given
a stale-pr label and should be manually reviewed.'

[arvidn]

is this still necessary when you make the other types final?

[knst]

Yes, because final in other class mean just no more inheritance and it doesn't need its own virtual destructor. But above class/struct still needs so

[arvidn]

ok. I guess we already have other virtual functions in this class, and we do use it polymorphically. so this seems right.

[wallentx]

Hi @knst, our branch protection rules require that all commits be signed before merging. If you know how to retroactively sign your initial commit, we can proceed with this PR. If you aren't sure how to do that, I've also prepared this PR #371 to merge in, with your attribution listed as co-author.

Please let me know how you'd like me to continue 👍

[wallentx]

@knst I'm going to go ahead and close out this PR, and merge in your changes in #371