style: Add a StyleBuilder struct to avoid refcount and atomic CAS during the cascade.

This should fix most of the complaints that caused
https://bugzilla.mozilla.org/show_bug.cgi?id=1360889 to be open, and also fix a
bunch of other FIXMEs across the style system.

components/style/properties/helpers.mako.rs

@@ -476,7 +476,7 @@
                     SpecifiedValue::Keyword(v) => v,
                     SpecifiedValue::System(_) => {
                         % if product == "gecko":
-                            _cx.style.cached_system_font.as_ref().unwrap().${to_rust_ident(name)}
+                            _cx.cached_system_font.as_ref().unwrap().${to_rust_ident(name)}
                         % else:
                             unreachable!()
                         % endif