Improve SPARQL serialization with support for IRI namespaces

[kasei]

Allow serializing SPARQL prefix names when a namespace map is passed to Attean::API::SPARQLSerializable->as_sparql.

AtteanX::Serializer::SPARQL may be constructed with a namespaces namespace map attribute (which comes from Attean::API::AbbreviatingSerializer). This will cause IRI tokens to be turned into prefix names during serialization. Based on that functionality, the Attean::API::SPARQLSerializable->as_sparql method should be updated to take a namespace map object (or possibly a hashref), and pass it to the AtteanX::Serializer::SPARQL->new constructor.

This should be a straightforward and simple patch.

[kasei]

Update: #53 made part of this issue redundant. I've updated the issue description, and this comment can now be ignored as well.

Commit 6cd0674 shows how this was done for the Turtle serializer. This can be mimicked in AtteanX::Serializer::SPARQL.