fast variadic string-concat

Hello,
often string functions of the form
str = str + concat(strings...)
or
str = concat(strings...)
are needed. But so far there is no variadic std::concat or boost::algorithm::concat. I would like to make up for that here.
The goal of course must be to limit memory operations to the most necessary, i.e. to avoid temporary strings and thus (superfluous) new/delete etc.
My current implementations [concat.hpp.txt](https://github.com/boostorg/algorithm/files/11763557/concat.hpp.txt)
This also works with very old compilers - of course C++11 is generally required https://godbolt.org/z/cvWrzxr1e

thx
Gero

Begin with the attached concat.hpp.txt implementation and compare its proposed API with the existing Boost.Algorithm structure. The issue names no repository file or test, so first locate the relevant algorithm entry points and test coverage. Done means an agreed variadic concat design with tests demonstrating the required behavior and memory-efficiency expectations.

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