When providing a blanket implmentation of T also optionally provide an appropriate `as_boxed` method

Let's say I have a `Box` that implements `T`, and some function `fn f(...) -> impl T`. `f` can return a `Box`, because `Box` implements `T`. If the caller needs to put the return `t` in a box, then the caller has to do: `Box::new(t)`. That's unfortunate, because `t` was actually already a `Box`, we just didn't know (or rather, couldn't exploit the fact).

A solution is to add a method to `T`: `fn as_boxed(self) -> Box`. The implementation on `T` would do: `return Box::new(self)`, and the implementation on `Box` would do: `return self`. No double boxing. This is what we use in Sequoia [1](https://gitlab.com/sequoia-pgp/sequoia/-/blob/main/buffered-reader/src/lib.rs#L851), [2](https://gitlab.com/sequoia-pgp/sequoia/-/blob/main/buffered-reader/src/lib.rs#L1003) where we ended up with an object being boxed millions of time.

As far as I can tell, blanket wraps all functions. So, we can't use blanket in this case. I'd like an option to either not wrap particular functions, or a mechanism to add an `as_boxed` method. As I can't imagine other functions that shouldn't be wrapped, this point solution is probably sufficient.