Add Value trait impls for ValueArray

[sdroege]

Mean problem here is that it allows circumventing the exclusivity of &mut references as it is actually reference counted in GValues. Needs some thinking!

[sdroege]

See https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/issues/274 for use-case

[sdroege]

Actually it implements the trait but it's hidden from the docs. @GuillaumeGomez can we not hide these trait impls that users would use? :)

Nonetheless there's the mutability problem I mentioned above.

[GuillaumeGomez]

Why would they be hidden?

[sdroege]

Because https://github.com/gtk-rs/glib/blob/5dfc0b5a986caaf687b5cc1648432fe4c73d0ad1/src/boxed.rs#L267

[GuillaumeGomez]

I'm fine with removing the #[doc(hidden)]. Do you plan to remove these from all traits?

[sdroege]

Only for the ones that are relevant for non-FFI usage. I would keep them for all the From/To pointer traits.

[GuillaumeGomez]

Then sounds good to me.

[sdroege]

Ok, well in addition we have to fix the soundness issue related to ValueArray specifically :)