Feature: specializations to reduce dispatch overhead of polymorphic_allocator

[liss-h]

This PR adds specializations for polymorphic_allocator<T, Alloc> and polymorphic_allocator<T, Alloc1, Alloc2> that aim to reduce dispatch overhead of the function calls.

TODOs:

• How does std::variant implement operator=?

[bigerl]

thx for the PR. I hope I didn't overlook anything in the verbosity of stl template stuff. Please find my comments below.

[liss-h]

How does std::variant implement operator=?

It doesn't use union, so whatever it does, doesn't really apply here

[liss-h]

So: any further ideas how to make operator= and swap less ugly? 😅

[bigerl]

So: any further ideas how to make operator= and swap less ugly? 😅

not at all.

but personally I don't find it too bad :)