Register mark objects so Protobuf is compaction friendly

This commit removes an unused reference and registers globals with the
GC so that they will never die.  Ruby is getting a compacting GC, and it
means that these references can move.  Registering them with
`rb_gc_register_mark_object` will ensure the constants don't move and
will not be collected.

ruby/ext/google/protobuf_c/protobuf.c

@@ -37,7 +37,7 @@
 #include "message.h"
 #include "repeated_field.h"
 
-VALUE cError;
+VALUE cParseError;
 VALUE cTypeError;
 
 const upb_fielddef* map_field_key(const upb_fielddef* field) {
@@ -368,8 +368,10 @@ void Init_protobuf_c() {
   Map_register(protobuf);
   Message_register(protobuf);
 
-  cError = rb_const_get(protobuf, rb_intern("Error"));
+  cParseError = rb_const_get(protobuf, rb_intern("ParseError"));
+  rb_gc_register_mark_object(cParseError);
   cTypeError = rb_const_get(protobuf, rb_intern("TypeError"));
+  rb_gc_register_mark_object(cTypeError);
 
   rb_define_singleton_method(protobuf, "discard_unknown",
                              Google_Protobuf_discard_unknown, 1);