der: use Cow for *Ref types

[dishmaker]

Following #1778 (comment),
we need to investigate why Cow<*Ref> does not work.

One major issue is that Clone is implemented on Ref types.

formats/der/src/asn1/ia5_string.rs

Lines 37 to 40 in 217b6ef

	#[derive(Copy, Clone, Eq, PartialEq, PartialOrd, Ord)]
	pub struct Ia5StringRef<'a> {
	/// Inner value
	inner: StrRef<'a>,

which collides with (if i remember correctly):
https://doc.rust-lang.org/std/borrow/trait.ToOwned.html#impl-ToOwned-for-T

[tarcieri]

Yeah, we'd need to override ToOwned, which would require dropping Copy/Clone which seems annoying.

Part of the problem is those types are owned wrappers around references, where typically types you Borrow only exist as references (e.g. str, [T], Path, where the references are trivially copyable), and ToOwned gets you back to an owned type, so it's not important to impl Copy/Clone.

One way this could potentially be fixed, with considerable effort, would be to turn the *Ref types into repr(transparent) wrappers for types like [u8] and str which are always accessed as e.g. &'a Ia5StringRef rather than Ia5StringRef<'a>, which would permit a proper Borrow/ToOwned relationship to e.g. Ia5String. This only works for "fat pointer" types though, and we have more complex *Ref types which are structs with a lifetime that have multiple fields that borrow.

[dishmaker]

These types can be repr(transparent). We can remove precomputed length (and ensure it's valid in all constructors).

BytesRef, StrRef as well as
GeneralStringRef, OctetStringRef, ... , Ia5StringRef, Utf8StringRef, VideotexStringRef

even BitStringRef can be repr(transparent) in a hacky way.

[dishmaker]

If we add DerCow alongside borrow::Cow, we can use #[cfg(feature = "alloc")] on the Owned variant:

pub enum DerCow<'a, B>
where
    B: RefToOwned<'a> + ?Sized,
{
    /// referenced, for example `OctetStringRef`
    Borrowed(&'a B),

    #[cfg(feature = "alloc")]
    /// allocated, for example `OctetString`
    Owned(<B as RefToOwned<'a>>::Owned),
}

https://github.com/RustCrypto/formats/pull/1778/files#diff-0029830a1b28345f613a09280075debe634157deea177c7535ba351380c2c8fdR62