Merge MoveAssignableVariant and CopyAssignableVariant
Merge MoveConstructibleVariant and CopyConstructibleVariant
Remove ConditionallyConstructibleVariant
Merge AssignableVariant into ConstructibleVariant
Move copy/move constructor to VariantBase
Rename ConstructibleVariant -> AssignableVariant
Revise assignment operator
Move emplace to VariantBase
Test create function and add create overload
Comment
Comment and cosmetic
Fix create function noexcept
Add of function