Substantial rewrites.

There's been much discussion at recent Rust lang team meetings about this
feature, followed by further discussion on Zulip and face-to-face at
RustNation.

The main conclusions have been:
* yes we want to do this; Rust for Linux has equally important use-cases
* the previously proposed deshadowing algorithm is believed to be sound but is
  also complex and counterintuitive, so we may want to more broadly rethink
  method resolution.
* so, for now, let's do the most conservative possible version - NOT supporting
  raw pointers, Weak or NonNull - but instead erroring on any case where there
  is a possible method conflict between an outer smart pointer type and its
  contained type. This will give us maximal flexibility to relax restrictions
  in future.

This PR updates the RFC correspondingly.

Prototype code can be found at:
rust-lang/rust@master...adetaylor:rust:receiver_trait_with_target_simplified_per_rustnation