Fix quadratic behavior with smart quotes

Make sure to remove matching smart quote delimiters. Otherwise, the
same opener could be found over and over again, preventing the
`openers_bottom` optimization from kicking in and leading to quadratic
behavior when processing lots of quotes.

Fixes commonmark#388.

src/inlines.c

@@ -698,18 +698,22 @@ static void process_emphasis(subject *subj, delimiter *stack_bottom) {
         } else {
           closer = closer->next;
         }
-      } else if (closer->delim_char == '\'') {
-        cmark_node_set_literal(closer->inl_text, RIGHTSINGLEQUOTE);
-        if (opener_found) {
-          cmark_node_set_literal(opener->inl_text, LEFTSINGLEQUOTE);
+      } else if (closer->delim_char == '\'' || closer->delim_char == '"') {
+        if (closer->delim_char == '\'') {
+          cmark_node_set_literal(closer->inl_text, RIGHTSINGLEQUOTE);
+        } else {
+          cmark_node_set_literal(closer->inl_text, RIGHTDOUBLEQUOTE);
         }
         closer = closer->next;
-      } else if (closer->delim_char == '"') {
-        cmark_node_set_literal(closer->inl_text, RIGHTDOUBLEQUOTE);
         if (opener_found) {
-          cmark_node_set_literal(opener->inl_text, LEFTDOUBLEQUOTE);
+          if (old_closer->delim_char == '\'') {
+            cmark_node_set_literal(opener->inl_text, LEFTSINGLEQUOTE);
+          } else {
+            cmark_node_set_literal(opener->inl_text, LEFTDOUBLEQUOTE);
+          }
+          remove_delimiter(subj, opener);
+          remove_delimiter(subj, old_closer);
         }
-        closer = closer->next;
       }
       if (!opener_found) {
         // set lower bound for future searches for openers