Attempt to return impl Element from render in Render trait

[nathansobo]

This PR uses the new Rust 1.75 "impl trait in trait" feature to simplify the Render trait.

However, I'm getting a borrow checker error that doesn't make sense to me. If anybody else wants to take a look, maybe they can figure it out. Not sure if it's a bug in the new feature.

Release Notes:

• N/A

[osiewicz]

It looks like the borrow checker works as intended in this case (sadly), as the returned impl Trait seems to be capturing all of the lifetimes in the scope implicitly (per this issue).
I've trimmed down this case to a repro, we'll see if it can be made to work somehow.

Edit: I think we'll have to wait for type alias impl trait per this design doc
Specifically this example goes over how it'd help us solve the issue of overcapturing.