Pre-RFC: `#[derive(From)]` for newtypes

[Nemo157]

@Kobzol

I propose to enable #[derive(From)] in a single place - when used on a tuple struct with a single field. In this case, it is 100% clear what should the compiler do, and there is (in my opinion) no question about how should such a derive be expanded.

I see at least two ways to expand it impl From<Inner> for Wrapper and impl<T> From<T> for Wrapper where Inner: From<T>