Mark swap as noexcept

[Flamefire]

All used operations are trivial or already noexcept so set the annotation.
This is recommended for swap implementations

[nigels-com]

std::swap(std::vector) is noexcept for C++17. But it's less clear for C++11 and C++14.

As a workaround, perhaps conditional compilation using BOOST_CXX_VERSION which is probably OK considering it's a header-only library?

#if BOOST_CXX_VERSION >= 201703L
// C++17 or later
#endif