Implement SerdeDiff for a few infallible container types

[AaronFriel]

Cow, Box, and Cell all have trivial implementations that should make it easy to support more downstream uses of SerdeDiff.

Rc, Arc, RefCell, and friends have either fallible read or write operations, so those are not implemented as those implementations would be opinionated, and I don't have an opinion on how that should be done.

Lastly, 'a in the derive macro is replaced by 'sd, mimicking the 'de from Serde's Deserialize.