Fix use of type objects in parsers, serializers, and iterators

[kasei]

Attean->acceptable_parsers(handles => 'Attean::API::TripleOrQuad') should include all triple parsers, but right now only includes n-quads. This is caused by brittle use of type objects and role strings to do the type checking of what parsers and iterators produce, and serializers consume.

item_type and handled_type shouldn't assume that the types are either role strings, or Type::Tiny::Role objects. Instead, look into having them all use Type::Tiny objects, including Type::Tiny::Union. Then update the code in acceptable_parsers to use Type::Tiny's API to check that $pclass->handled_type (triples) is a subset of the preferred type (triples and quads).