Ref https://github.com/paritytech/ci_cd/issues/1107

We mainly need that so that we can finally compile the `pallet_revive`
fixtures on stable. I did my best to keep the commits focused on one
thing to make review easier.

All the changes are needed because rustc introduced more warnings or is
more strict about existing ones. Most of the stuff could just be fixed
and the commits should be pretty self explanatory. However, there are a
few this that are notable:

## `non_local_definitions `

A lot of runtimes to write `impl` blocks inside functions. This makes
sense to reduce the amount of conditional compilation. I guess I could
have moved them into a module instead. But I think allowing it here
makes sense to avoid the code churn.

## `unexpected_cfgs`

The FRAME macros emit code that references various features like `std`,
`runtime-benchmarks` or `try-runtime`. If a create that uses those
macros does not have those features we get this warning. Those were
mostly when ...