ReferenceAccessor should be a factory

[SebastianStehle]

The reference accessor is actually super weird. First, an object is created. Then we check the handle and discard this object again. This puts unnecessary pressure on the GC.

I think we should just rename it to ReferenceFactory and refactor it to methods like this:

public static Output? Output(nint handle, bool shouldDispose)
{
     return handle == nint.Zero ? null : new Output(handle, shouldDispose);
}

Depending on #48 it might event be obsolete.

Please assign me, if you agree. Happy to provide a PR.

[LSViana]

Hi, @SebastianStehle!

I like this idea. I'm not completely happy with ReferenceAccessor because it's just static code to centralize some operations but I didn't put a lot of thought here. It could be better.

However, let's finish #48 first then we discuss what to do here.