der: introduce OwnedtoRef/RefToOwned traits

[baloo]

This introduces the two new traits to go from a ref object to an owned object as discussed in #796

closes#796

[baloo]

Guess I'll need to gate those to v1.65 since I'm making use of GAT (didn't even realize that)

[tarcieri]

MSRV bump to 1.65 seems aggressive but probably fine as 1.66 will be out momentarily and it will still be several months before these crates are released

[baloo]

I think I like the rustversion macro (https://github.com/dtolnay/rustversion) a lot for that.
The OwnerToRef is gated for >=1.65 but we retain the 1.60 MSRV just without the trait.

[baloo]

I like the situation though:
https://github.com/RustCrypto/formats/actions/runs/3701100883/jobs/6270113493#step:4:92

The syntax moved from (1.60)

 type Borrowed<'a>
where
T: 'a,
= Option<T::Borrowed<'a>>;

to (at least 1.65)

type Borrowed<'a> = Option<T::Borrowed<'a>> where T: 'a;

and it's now a warning.

[tarcieri]

I’m fine with bumping the MSRV outright. I’d prefer to avoid additional complexity just to preserve an arbitrary MSRV.

[tarcieri]

@baloo can you rebase?

[tarcieri]

D'oh, got the MSRV wrong in the commit message. Mea culpa.