using keyword combined with discard in out params

[henke37]

The using keyword is awesome, especially when working with native objects that would normally need a finalizer.

The discard "variable" is great when working with out parameters that you don't care about.

However, you can't combine the two to elegantly dispose an object that you just want to discard.

It'd be cool if you could write a line like this:

Fun(using out _);

[HaloFour]

See: #3544