csharp: Implement Deconstruct for structs.

csharp: Implement Deconstruct for structs.

Summary:
Usage example:

```
var (x, y) = somePos2D;
```

Not available on Mono environment due to [`mcs` not implementing
it](https://github.com/mono/mono/blob/a3de0304a190c54124df3486ce9c34b6262787a8/mcs/mcs/tuples.cs#L590).
To keep tests from breaking because of it, a `MONO` preprocessor variable is
defined and checked during test compilation.

Ref T8489.

Reviewers: brunobelo, lauromoura, segfaultxavi

Reviewed By: lauromoura

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Maniphest Tasks: T8489

Differential Revision: https://phab.enlightenment.org/D10809