`std::is_assignable_v<Dest&, Source>` should be wrapped in concept `X4Assignable`

[saki7]

MSVC usually omits the expansion of actual template arguments when the plain metafunction is used. Using concept somehow enforces MSVC to always emit the actual types, which is a great QoL improvement.

Plain std::is_assignable_v<Dest&, Source> (current main branch)

With X4Assignable

[saki7]

Note: std::assignable_from is NOT the same as std::is_assignable_v and cannot be used for this purpose.

[saki7]

This can be fixed any time, but I think the changes would conflict with #61 and #42 so I'm waiting for those PRs to be finished