[REG2.065a] Issue 12071 - Algebraic won't take delegate returning structure

https://d.puremagic.com/issues/show_bug.cgi?id=12071

Use `Variant` rather than `VariantN` in order to pack arguments and returned value in opCall.

Note that, this is rough fix. Maybe we should strictly calculate opCall return type for the `Algebraic` instances?

Assign to: @andralex