Auto merge of #46862 - nikomatsakis:nll-master, r=arielb1

NLL feature complete (adds `feature(nll)`)!

This is the final PR for the nll-master branch; it brings over all remaining content.

The contents of the branch include:

- track causal information and use it to report extended errors
- handle `impl Trait` in NLL code
- improve printing of outlives errors
- add `#![feature(nll)]` and some more sample tests

The commits should for the most part build independently.

r? @pnkfelix (and/or @arielb1)