Generate associated constants indicating serialized size for *Report types

[CobaltCause]

To implement this, a new trait like this could be added:

pub trait ReportSize {
    const BYTES_COUNT: usize;
}

and then an implementation of this trait is generated for report types automatically by #[gen_hid_descriptor]. I'm not sure if there needs to be two associated constants, one for input and output, or if they're always the same size and so one associated constant would suffice.

This would be useful for automatically statically allocating buffers via the type system instead of having to hard-code magic numbers.

[twitchyliquid64]

Thats a great idea!

[HaoboGu]

I have the same issue, having ReportSize would be much helpful. @twitchyliquid64 @CobaltCause Do you have any plan for adding this?