Fix clippy::drop_ref lint

Clippy doesn't like calls to `std::mem::drop` with references --- since
the pointed value is not actually dropped. In this case, this *is* the
correct behavior, but clippy views this as a footgun if the user means
to actually drop the value behind the reference.

Here, we're using `drop` to ignore values in no-op default impls
for trait methods.  I've
changed those methods to use `let _` to ignore parameters instead. This
doesn't trigger the clippy lint and is maybe more idiomatic anyway.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

valuable/src/visit.rs

@@ -223,7 +223,7 @@ pub trait Visit {
     /// valuable::visit(&my_struct, &mut Print);
     /// ```
     fn visit_named_fields(&mut self, named_values: &NamedValues<'_>) {
-        drop(named_values);
+        let _ = named_values;
     }
 
     /// Visit a struct or enum's unnamed fields.
@@ -267,7 +267,7 @@ pub trait Visit {
     /// valuable::visit(&my_struct, &mut Print);
     /// ```
     fn visit_unnamed_fields(&mut self, values: &[Value<'_>]) {
-        drop(values);
+        let _ = values;
     }
 
     /// Visit a primitive slice.
@@ -359,7 +359,7 @@ pub trait Visit {
     /// valuable::visit(&map, &mut Print);
     /// ```
     fn visit_entry(&mut self, key: Value<'_>, value: Value<'_>) {
-        drop((key, value));
+        let _ = (key, value);
     }
 }