der: integrate `Document` type with `yoke`?

[tarcieri]

The yoke crate implements support for attaching zero-copy deserialized data types (i.e. ones with lifetimes that borrow from the original encoded data) to the data they're decoded from:

https://docs.rs/yoke/

The der::Document type provides backing storage for an ASN.1 DER-encoded document. It might be interesting to (optionally) integrate Yoke or a newtype thereof as a way to parse and cache a zero-copy data type which borrows from a Document.