der: add class bits consts for Application and Private tag support

[dishmaker]

It's needed for generic Application, ContextSpecific, Private tags.

My old PR:

• der: Generic custom-class long > 30 tags PRIVATE, CONTEXT-SPECIFIC, APPLICATION #1545

This PR contains der/src/tag/class.rs

.bits() will be used in generic wrappers like this:

impl<'a, const TAG: u32, T, const CLASS: u8> Choice<'a> for CustomClassExplicit<TAG, T, CLASS>
where
    T: Decode<'a> + Tagged,
{
    fn can_decode(tag: Tag) -> bool {
        tag.class().bits() == CLASS && tag.number() == TagNumber(TAG)
    }
}

and consts like this:

/// Application class wrapper, EXPLICIT
pub type ApplicationExplicit<const TAG: u32, T> = CustomClassExplicit<TAG, T, CLASS_APPLICATION>;