wip: add der::Cow enum

[dishmaker]

Use case:

#[derive(Clone, Debug, Eq, PartialEq, Sequence, ValueOrd)]
#[allow(missing_docs)]
pub struct TbsCertificateInnerCow<'a, P: Profile = Rfc5280> {
    /// The certificate version.
    ///
    /// Note that this value defaults to Version 1 per the RFC. However,
    /// fields such as `issuer_unique_id`, `subject_unique_id` and `extensions`
    /// require later versions. Care should be taken in order to ensure
    /// standards compliance.
    #[asn1(context_specific = "0", default = "Default::default")]
    pub(crate) version: Version,

    pub(crate) serial_number: SerialNumber<P>,
    pub(crate) signature: AlgorithmIdentifier,
    pub(crate) issuer: NameCow<'a>,
    pub(crate) validity: Validity<P>,
    pub(crate) subject: NameCow<'a>,
    pub(crate) subject_public_key_info: SubjectPublicKeyInfoCow<'a>,

    #[asn1(context_specific = "1", tag_mode = "IMPLICIT", optional = "true")]
    pub(crate) issuer_unique_id: Option<DerCow<BitStringRef<'a>>>,

    #[asn1(context_specific = "2", tag_mode = "IMPLICIT", optional = "true")]
    pub(crate) subject_unique_id: Option<DerCow<BitStringRef<'a>>>,

    #[asn1(context_specific = "3", tag_mode = "EXPLICIT", optional = "true")]
    pub(crate) extensions: Option<ext::ExtensionsCow<'a>>,
}

[dishmaker]

I have a skill issue

[baloo]

I don't know how I feel about this one. I think I would rather duplicate the types to add a *Ref<'a> rather than rely on Cow.

(not a suggestion, or a request to change this PR at this moment, just a vibe)

[tarcieri]

I'd agree this probably doesn't make sense, and would suggest potentially investigating trying to impl the relevant traits to use alloc::borrow::Cow (though I'm not sure &'a TypeRef<'a> would actually work for e.g. Borrow?)