This patch implements
<https://lichray.github.io/trivially_variant.html>, but with more
changes than the proposal's. It
1) Creates __detail::__variant::_Traits as a centralized place to hold
common (but not all yet) compile-time conditions.
2) Changes the noexcept conditions for the (copy|move) (ctor|assign)
SMFs, so that when one is trivial, one is also noexcept. It's not the
same as p0088r3, nor p0088r3 + D0602R1 anymore.
3) Creates 4 structs, namely (_Copy|_Move)_(ctor|assign)_(base|alias)
for dispatch on triviality. The code that were originally in
_Variant_base are moved into these four structs. There aren't
functional changes except for more triviality.

Sorry for having a large patch. Do tell me if you want me to split it.

Tested on x86_64-linux-gnu.

Thanks!


-- 
Regards,
Tim Shen