More refactoring

I added support of XML::Struct, but PICAXML, MAB etc. should better be put into their own modules, e.g. Catmandu::PICA should include Catmandu::SRU::Importer::Parser::picaxml and Catmandu::MARC should include Catmandu::SRU::Importer::Parser::marcxml. There is no need to add a dependency to Catmandu::SRU because the Catmandu::SRU::Importer::Parser::* modules use duck-typing instead of deriving from Catmandu::SRU::Importer::Parser.