excise duplicate code from marshal.py
excise duplicate code from marshal.py

lots simpler just to put the struct.Struct check in just one place, and
abstract the repeating pattern into a maker function.

EnigmaCurry/cassandra-dbapi2,CanWeStudio/cassandra-dbapi2,maraca/cassandra-dbapi2,iflatness/cql,iGivefirst/cassandra-dbapi2,pcmanus/python-cql,pandu-rao/cassandra-dbapi2,josh-mckenzie/cassandra-dbapi2,diezguerra/cassandra-dbapi2,flowroute/cassandra-dbapi2,stanhu/cassandra-dbapi2,mshuler/cassandra-dbapi2