Register all global VALUES as mark objects

Previously it was possible for these objects to be garbage collected
(the constants could be unassigned in Ruby) or moved as part of GC
compaction (reproducible with GC.verify_compaction_references).

This commit marks all the global variables in nokogumbo.c using
rb_gc_register_mark_object to ensure that they can't be moved.

ext/nokogumbo/nokogumbo.c

@@ -724,10 +724,15 @@ void Init_nokogumbo() {
   VALUE mNokogiri = rb_const_get(rb_cObject, rb_intern_const("Nokogiri"));
   VALUE mNokogiriXml = rb_const_get(mNokogiri, rb_intern_const("XML"));
   cNokogiriXmlSyntaxError = rb_const_get(mNokogiriXml, rb_intern_const("SyntaxError"));
+  rb_gc_register_mark_object(cNokogiriXmlSyntaxError);
   cNokogiriXmlElement = rb_const_get(mNokogiriXml, rb_intern_const("Element"));
+  rb_gc_register_mark_object(cNokogiriXmlElement);
   cNokogiriXmlText = rb_const_get(mNokogiriXml, rb_intern_const("Text"));
+  rb_gc_register_mark_object(cNokogiriXmlText);
   cNokogiriXmlCData = rb_const_get(mNokogiriXml, rb_intern_const("CDATA"));
+  rb_gc_register_mark_object(cNokogiriXmlCData);
   cNokogiriXmlComment = rb_const_get(mNokogiriXml, rb_intern_const("Comment"));
+  rb_gc_register_mark_object(cNokogiriXmlComment);
 
   // Interned symbols.
   new = rb_intern_const("new");
@@ -740,6 +745,7 @@ void Init_nokogumbo() {
   // Class constants.
   VALUE HTML5 = rb_const_get(mNokogiri, rb_intern_const("HTML5"));
   Document = rb_const_get(HTML5, rb_intern_const("Document"));
+  rb_gc_register_mark_object(Document);
 
   // Interned symbols.
   internal_subset = rb_intern_const("internal_subset");