Rewrite with future-compat lint for indirect pattern omitting

`#[structural_match]`.

Outline of changes:

 * Recur as deeply as necessary when searching for `#[structural_match]`.

 * `#[structural_match]`: handle case of `const A: & &Wrap(NoDerive)`
   by including the fields of an ADT during traversal of input
   type. (We continue to not traverse the substs of an ADT, though, so
   that we continue to handle `PhantomData<NoDerive>` and `*NoDerive`
   properly.)

 * Refactored code to use `match` instead of `if let`. This ends up
   *with less* right-ward drift by moving the handling of the main
   *`ty::Adt` case *outside* the match.

 * Using lint (rather than hard error) mmeans we need to check that
   type is `PartialEq` to avoid ICE'ing the compiler in scneario where
   MIR codegen dispatches to `PartialEq::eq`. Added said check, and
   fatal error in that case.

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