Improved perf of XPath::Evaluator#node_matches?

Using the benchmark xpath/evaluator/node_matches_bench.rb the results
prior to this commit were as following for 3 cases:

    name only:          737633 i/s
    namespace wildcard: 612196 i/s
    name wildcard:      516030 i/s

With this commit said numbers have changed to the following:

    name only:          746086  i/s
    namespace wildcard: 1097168 i/s
    name wildcard:      1151255 i/s

This results in the following increase of performance for each case:

    name only:          1,011x (insignificant)
    namespace wildcard: 1,79x
    name wildcard:      2,23x

In the benchmark xpath/evaluator/big_xml_average_bench.rb the difference
isn't really noticable as said benchmark only queries elements by names,
of which the performance hasn't really improved.

lib/oga/xpath/evaluator.rb

@@ -59,6 +59,9 @@ module Oga
     #     evaluator.evaluate('$number') # => 10
     #
     class Evaluator
+      # Wildcard for node names/namespace prefixes.
+      STAR = '*'
+
       ##
       # @param [Oga::XML::Document|Oga::XML::Node] document
       # @param [Hash] variables Hash containing variables to expose to the XPath
@@ -534,7 +537,7 @@ module Oga
           next unless context_node.respond_to?(:available_namespaces)
 
           context_node.available_namespaces.each do |_, namespace|
-            if namespace.name == name or name == '*'
+            if namespace.name == name or name == STAR
               nodes << namespace
             end
           end
@@ -1643,8 +1646,8 @@ module Oga
 
         # If only the name is given and is a wildcard then we'll also want to
         # match the namespace as a wildcard.
-        if !ns and name == '*'
-          ns = '*'
+        if !ns and name == STAR
+          ns = STAR
         end
 
         name_matches = name_matches?(xml_node, name)
@@ -1686,7 +1689,7 @@ module Oga
       def name_matches?(xml_node, name)
         return false unless xml_node.respond_to?(:name)
 
-        return xml_node.name == name || name == '*'
+        return name == STAR ? true : xml_node.name == name
       end
 
       ##
@@ -1699,7 +1702,7 @@ module Oga
       def namespace_matches?(xml_node, ns)
         return false unless xml_node.respond_to?(:namespace)
 
-        return xml_node.namespace.to_s == ns || ns == '*'
+        return ns == STAR ? true : xml_node.namespace.to_s == ns
       end
 
       ##