Make static_vector<T,N> trivially relocatable when T is trivially relocatable

[Quuxplusone]

This PR definitely isn't ready for prime time, but I figured I might as well exploit Cunningham's Law here.

The idea is that when the compiler supports __cpp_impl_trivially_relocatable (P1144's proposed feature-test macro), then we should define -DBOOST_CONTAINER_ATTRIBUTE_TRIVIALLY_RELOCATABLE_IF(x)=[[trivially_relocatable(x)]]. (And/or set it to a vendor-specific attribute if-and-when anyone supports a vendor-specific attribute; no mainstream compiler does, yet.)

The big problem with this patch as it stands right now is that I have no idea where these macros are configured. We want to do the same thing that BOOST_CONTAINER_ATTRIBUTE_NODISCARD does; but that's not configured anywhere in this repo, either.

I see that Boost.Container is just inheriting all its type-traits from Boost.Move, so probably dtl::is_trivially_relocatable belongs in Boost.Move, not here?

Fixes #258.