Correctly move callable `f` in `Unwrapping` call operator

[msimberg]

The previous implementation was potentially buggy for cases when the Unwrapping call operator is called more than once, since f was for the actual call.

As far as I could tell we're not actually affected by this bug since we only ever call the Unwrapping call operator once.

This splits the call operator into two && and & qualified overloads, with the latter not moving f. Note that the latter is not const& because that would fail with wrapped call operators that are not const-qualified. The Duplicate call operator was one such call operator. I've changed the latter to be const-qualified but I've also left the Unwrapping call operator as & in case we introduce other call operators that actually need to be non-const.

@aurianer you can apply the same change in #930 to ConsumeRvalues.

[msimberg]

cscs-ci run

[msimberg]

cscs-ci run