Wrap ParseArgs in a Ruby object

The functions passed to `rb_ensure` should be `VALUE`s. It's probably
the case that Ruby is going to pass these directly without ever
examining them, but maybe not.

This wraps the `ParseArgs` pointer as a Ruby object (of class `Data`
which is what the documentation seems to suggest rather than class
`Object`, but `Data` is also deprecated so who knows?).

This fixes several compilation warnings about mismatched types.

ext/nokogumbo/nokogumbo.c

@@ -480,15 +480,38 @@ typedef struct {
   xmlDocPtr doc;
 } ParseArgs;
 
-static VALUE parse_cleanup(ParseArgs *args) {
+static void parse_args_mark(void *parse_args) {
+  ParseArgs *args = parse_args;
+  rb_gc_mark_maybe(args->input);
+  rb_gc_mark_maybe(args->url_or_frag);
+}
+
+// Wrap a ParseArgs pointer. The underlying ParseArgs must outlive the
+// wrapper.
+static VALUE wrap_parse_args(ParseArgs *args) {
+  return Data_Wrap_Struct(rb_cData, parse_args_mark, RUBY_NEVER_FREE, args);
+}
+
+// Returnsd the underlying ParseArgs wrapped by wrap_parse_args.
+static ParseArgs *unwrap_parse_args(VALUE obj) {
+  ParseArgs *args;
+  Data_Get_Struct(obj, ParseArgs, args);
+  return args;
+}
+
+static VALUE parse_cleanup(VALUE parse_args) {
+  ParseArgs *args = unwrap_parse_args(parse_args);
   gumbo_destroy_output(args->output);
+  // Make sure garbage collection doesn't mark the objects as being live based
+  // on references from the ParseArgs. This may be unnecessary.
+  args->input = Qnil;
+  args->url_or_frag = Qnil;
   if (args->doc != NIL)
     xmlFreeDoc(args->doc);
   return Qnil;
 }
 
-
-static VALUE parse_continue(ParseArgs *args);
+static VALUE parse_continue(VALUE parse_args);
 
 // Parse a string using gumbo_parse into a Nokogiri document
 static VALUE parse(VALUE self, VALUE input, VALUE url, VALUE max_attributes, VALUE max_errors, VALUE max_depth) {
@@ -504,10 +527,13 @@ static VALUE parse(VALUE self, VALUE input, VALUE url, VALUE max_attributes, VAL
     .url_or_frag = url,
     .doc = NIL,
   };
-  return rb_ensure(parse_continue, (VALUE)&args, parse_cleanup, (VALUE)&args);
+  VALUE parse_args = wrap_parse_args(&args);
+
+  return rb_ensure(parse_continue, parse_args, parse_cleanup, parse_args);
 }
 
-static VALUE parse_continue(ParseArgs *args) {
+static VALUE parse_continue(VALUE parse_args) {
+  ParseArgs *args = unwrap_parse_args(parse_args);
   GumboOutput *output = args->output;
   xmlDocPtr doc;
   if (output->document->v.document.has_doctype) {
@@ -565,7 +591,7 @@ static xmlNodePtr extract_xml_node(VALUE node) {
 #endif
 }
 
-static VALUE fragment_continue(ParseArgs *args);
+static VALUE fragment_continue(VALUE parse_args);
 
 static VALUE fragment (
   VALUE self,
@@ -696,11 +722,13 @@ static VALUE fragment (
     .url_or_frag = doc_fragment,
     .doc = (xmlDocPtr)extract_xml_node(doc),
   };
-  rb_ensure(fragment_continue, (VALUE)&args, parse_cleanup, (VALUE)&args);
+  VALUE parse_args = wrap_parse_args(&args);
+  rb_ensure(fragment_continue, parse_args, parse_cleanup, parse_args);
   return Qnil;
 }
 
-static VALUE fragment_continue(ParseArgs *args) {
+static VALUE fragment_continue(VALUE parse_args) {
+  ParseArgs *args = unwrap_parse_args(parse_args);
   GumboOutput *output = args->output;
   VALUE doc_fragment = args->url_or_frag;
   xmlDocPtr xml_doc = args->doc;