Rename the ConstS::val field as kind.

And likewise for the `Const::val` method.

Because its type is called `ConstKind`. Also `val` is a confusing name
because `ConstKind` is an enum with seven variants, one of which is
called `Value`. Also, this gives consistency with `TyS` and `PredicateS`
which have `kind` fields.

The commit also renames a few `Const` variables from `val` to `c`, to
avoid confusion with the `ConstKind::Value` variant.

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -828,7 +828,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 }
                 ty::PredicateKind::ConstEquate(c1, c2) => {
                     let evaluate = |c: ty::Const<'tcx>| {
-                        if let ty::ConstKind::Unevaluated(unevaluated) = c.val() {
+                        if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
                             match select.infcx().const_eval_resolve(
                                 obligation.param_env,
                                 unevaluated,