Update ty::VariantDef to use IndexVec<FieldIdx, FieldDef>

And while doing the updates for that, also uses `FieldIdx` in `ProjectionKind::Field` and `TypeckResults::field_indices`.

There's more places that could use it (like `rustc_const_eval` and `LayoutS`), but I tried to keep this PR from exploding to *even more* places.

Part 2/? of https://github.com/rust-lang/compiler-team/issues/606

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -486,8 +486,7 @@ pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) -> Coe
             // U` can be coerced to `*mut V` if `U: Unsize<V>`.
             let fields = &def_a.non_enum_variant().fields;
             let diff_fields = fields
-                .iter()
-                .enumerate()
+                .iter_enumerated()
                 .filter_map(|(i, f)| {
                     let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));