style: Make MatchingContext generic over SelectorImpl.

This will help Xidorn implement tree pseudos, and in general makes sense,
allowing to put specific matching data in a selectors implementation.

components/selectors/context.rs

@@ -5,6 +5,7 @@
 use attr::CaseSensitivity;
 use bloom::BloomFilter;
 use nth_index_cache::NthIndexCache;
+use parser::SelectorImpl;
 use tree::OpaqueElement;
 
 /// What kind of selector matching mode we should use.
@@ -74,7 +75,10 @@ impl QuirksMode {
 /// Data associated with the matching process for a element.  This context is
 /// used across many selectors for an element, so it's not appropriate for
 /// transient data that applies to only a single selector.
-pub struct MatchingContext<'a> {
+pub struct MatchingContext<'a, Impl>
+where
+    Impl: SelectorImpl,
+{
     /// Input with the matching mode we should use when matching selectors.
     pub matching_mode: MatchingMode,
     /// Input with the bloom filter used to fast-reject selectors.
@@ -107,9 +111,13 @@ pub struct MatchingContext<'a> {
 
     quirks_mode: QuirksMode,
     classes_and_ids_case_sensitivity: CaseSensitivity,
+    _impl: ::std::marker::PhantomData<Impl>,
 }
 
-impl<'a> MatchingContext<'a> {
+impl<'a, Impl> MatchingContext<'a, Impl>
+where
+    Impl: SelectorImpl,
+{
     /// Constructs a new `MatchingContext`.
     pub fn new(
         matching_mode: MatchingMode,
@@ -144,6 +152,7 @@ impl<'a> MatchingContext<'a> {
             classes_and_ids_case_sensitivity: quirks_mode.classes_and_ids_case_sensitivity(),
             scope_element: None,
             nesting_level: 0,
+            _impl: ::std::marker::PhantomData,
         }
     }