Improve performance/usability of STL util `concat`
Improve performance/usability of STL util `concat`

It can now deal with heterogeneous buffer types (mainly std::vector
and Botan::secure_vector) and doesn't copy the data more than
necessary.

Co-Authored-By: Michael Boric <8a5c361d6b1af1a8b67e4841c80acaf60a883500@rohde-schwarz.com>

randombit/botan,randombit/botan,randombit/botan,randombit/botan,randombit/botan