style: Store values for :nth- selectors in dedicated struct NthSelectorData

To accomplish this, all :nth- Components were replaced with type Nth,
which uses NthSelectorData.

Using NthSelectorData will make it easier to add selector lists for :nth
selectors later on, but this change by itself shouldn't change any
behavior.

Differential Revision: https://phabricator.services.mozilla.com/D165831

components/malloc_size_of/lib.rs

@@ -717,10 +717,7 @@ where
             Component::Root |
             Component::Empty |
             Component::Scope |
-            Component::NthChild(..) |
-            Component::NthLastChild(..) |
-            Component::NthOfType(..) |
-            Component::NthLastOfType(..) |
+            Component::Nth(..) |
             Component::FirstOfType |
             Component::LastOfType |
             Component::OnlyOfType |