Borrow data in Option<Cow<str>> and Option<Cow<[u8]>> with serde(borrow)

[sffc]

Fixes #2016

Please see the bug for more information. Thank you in advance!

CC @Manishearth

[dtolnay]

I think this is going to be too niche to be worth building into the serde crate. There are infinite other combinations (like Vec<Cow<str>>) and my feeling is that the current place the line is drawn is appropriate.

There is an external crate that explores composable deserialize_with functions with a way to apply them inside of an Option, Vec etc, so they might be receptive to exposing borrow-based impls for Cow: https://docs.rs/serde_with/1.9.4/serde_with/attr.serde_as.html

[Manishearth]

Ooh, we weren't aware of the serde_with crate, that's neat!