Properly fixed AST for first-of-type/last-of-type

This requires keeping track of the current element being processed. This in turn
allows the usage of count() + preceding-sibling/following-sibling.

spec/oga/css/parser/pseudo_classes/first_of_type_spec.rb

@@ -4,13 +4,13 @@ describe Oga::CSS::Parser do
   context ':first-of-type pseudo class' do
     example 'parse the :first-of-type pseudo class' do
       parse_css(':first-of-type').should == parse_xpath(
-        'descendant::*[position() = 1]'
+        'descendant::*[count(preceding-sibling::*) = 0]'
       )
     end
 
     example 'parse the a:first-of-type pseudo class' do
       parse_css('a:first-of-type').should == parse_xpath(
-        'descendant::a[position() = 1]'
+        'descendant::a[count(preceding-sibling::a) = 0]'
       )
     end
   end

spec/oga/css/parser/pseudo_classes/last_of_type_spec.rb

@@ -4,7 +4,13 @@ describe Oga::CSS::Parser do
   context ':last-of-type pseudo class' do
     example 'parse the :last-of-type pseudo class' do
       parse_css(':last-of-type').should == parse_xpath(
-        'descendant::*[position() = last()]'
+        'descendant::*[count(following-sibling::*) = 0]'
+      )
+    end
+
+    example 'parse the a:last-of-type pseudo class' do
+      parse_css('a:last-of-type').should == parse_xpath(
+        'descendant::a[count(following-sibling::a) = 0]'
       )
     end
   end