Test RegexpRef type using SvRX or checking PERL_magic_qr

Test RegexpRef type using SvRX or checking PERL_magic_qr

With pluggable regexp engines checking "ref" of a qr// isn't correct;
it's also possible for a normal Regex to be blessed into another
class.

Additionally an 'Object' subtype previously excluded objects that were
->isa('Regexp'), this was inconsistent with the RegexpRef handling
(i.e. neither RegexpRef nor isa => 'Regexp' would accept a subclass of
Regexp). The 'Object' subtype now simply checks if the reference is
blessed with no special logic.