Use documented unittests for `std.traits.isAssignable`.
[Refactor] Remove redundant parantheses in `std.traits.isAssignable` unittest.
Clarify `isAssignable` documentation. Add private `isRvalueAssignable` and `isLvalueAssignable`.

Previously `isAssignable` worked like `isLvalueAssignable`.

Also add some unittests.
Merge pull request #1597 from denis-sh/fix-`std.traits.isAssignable`

Fix `std.traits.isAssignable`