Namespace.hs: added types for mappings and a prefix map.
Namespace.hs: added types for mappings and a prefix map.

darcs-hash:20071017013626-db1bb-8741e5b3db040df1bcefd639bdcaebac965d2df4

LeifW/rdf4h,cordawyn/rdf4h,jutaro/rdf4h,ddssff/rdf4h