Generator: ByRef generator now passes the value by ref

[Therzok]

This avoid an extra heap allocation for each object passed by ref so it can be transitioned directly into native memory. Native counterparts will handle data mutation, so managed will see it anyway.

https://gist.github.com/Therzok/f0627b87a5f89c1e3e5b2aab58b59b08

Don't merge yet, as I need to test it.

[Therzok]

Merging as it works for MD just fine.

[Therzok]

@knocte something worthy of backporting to master?

[knocte]

Well, in general, I think that's the wrong question to ask. Master should have all the nice things, and only certain things should be backported (so you should have said "forward-porting" not "backporting", here ;) ). That being said, I would really would like to have a 3.0 release from master before landing these things (as that has been stable for long), however I also would prefer to have that 2.12 release before the 3.0 one...