Move `trailers` submodule to `transfer`

[yoshuawuyts]

Trailers should be grouped together with TE and Transfer-Encoding headers.

[brightly-salty]

I'm willing to attempt to implement this and submit a PR. Do you want the path to Trailers to be crate::transfer::Trailers or crate::transfer::trailers::Trailers? I think it would be best to base it off of my remove-reexports branch (PR #305 )

[brightly-salty]

Never mind, I'll base it off of the transfer branch.