style: Clean up cascade rule iteration. r=nordzilla

The current API was pretty awkward as a result of two things:

 * Not being able to create empty iterators for smallbitvec.
 * We used to call the `F` function multiple times, but turns out that
   collecting the declarations in a SmallVec was a perf win.

So clean this up so that it looks more similar to other APIs, taking an
iterator directly.

This is a bit more code, but hopefully easier to understand (and also hopefully
easier to optimize).

The motivation for this work is that I plan to investigate rebasing / landing
https://github.com/servo/servo/pull/20151, and I don't want more instantiations
of apply_declarations and such.

Differential Revision: https://phabricator.services.mozilla.com/D74369

components/style/animation.rs

@@ -978,7 +978,13 @@ where
         } => {
             let guard = declarations.read_with(context.guards.author);
 
-            let iter = || {
+            // This currently ignores visited styles, which seems acceptable,
+            // as existing browsers don't appear to animate visited styles.
+            let computed = properties::apply_declarations::<E, _>(
+                context.stylist.device(),
+                /* pseudo = */ None,
+                previous_style.rules(),
+                &context.guards,
                 // It's possible to have !important properties in keyframes
                 // so we have to filter them out.
                 // See the spec issue https://github.com/w3c/csswg-drafts/issues/1824
@@ -986,17 +992,7 @@ where
                 guard
                     .normal_declaration_iter()
                     .filter(|declaration| declaration.is_animatable())
-                    .map(|decl| (decl, Origin::Author))
-            };
-
-            // This currently ignores visited styles, which seems acceptable,
-            // as existing browsers don't appear to animate visited styles.
-            let computed = properties::apply_declarations::<E, _, _>(
-                context.stylist.device(),
-                /* pseudo = */ None,
-                previous_style.rules(),
-                &context.guards,
-                iter,
+                    .map(|decl| (decl, Origin::Author)),
                 Some(previous_style),
                 Some(previous_style),
                 Some(previous_style),