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_middle/src/ty/error.rs

@@ -254,7 +254,7 @@ impl<'tcx> Ty<'tcx> {
                 }
 
                 let n = tcx.lift(n).unwrap();
-                if let ty::ConstKind::Value(v) = n.val() {
+                if let ty::ConstKind::Value(v) = n.kind() {
                     if let Some(n) = v.try_to_machine_usize(tcx) {
                         return format!("array of {} element{}", n, pluralize!(n)).into();
                     }