der: add AnyCow

[dishmaker]

AnyRef, that allocates as Any with alloc feature

[tarcieri]

I would prefer not to have any of these types that try to special case Cow.

We can likely do something similar to what I proposed in #1795 for Any/AnyRef using tagged pointers to store the tag for AnyRef

[tarcieri]

Hmm, one problem with tagged pointers is that the Tag type can now need up to 48-bits to represent, which can't be stored in e.g. a 32-bit tagged pointer

[dishmaker]

Yes, the Tag alongside &'a [u8] is a problem.

One parallel solution is:

pub struct AnyCow<'a> {
    tag: Tag,
    value: Cow<'a, BytesRef>,
}

but that wouldn't be alloc-gated on allocating variant.