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_monomorphize/src/collector.rs

@@ -1119,7 +1119,8 @@ fn find_vtable_types_for_unsizing<'tcx>(
             let target_fields = &target_adt_def.non_enum_variant().fields;
 
             assert!(
-                coerce_index < source_fields.len() && source_fields.len() == target_fields.len()
+                coerce_index.index() < source_fields.len()
+                    && source_fields.len() == target_fields.len()
             );
 
             find_vtable_types_for_unsizing(