wrap handler maps in a container to discourage modification
wrap handler maps in a container to discourage modification

We considered implementing Associative and related interfaces on
the HandlerMapContainer to support interrogation while suppressing
modification, but this is simpler and we don't have a real use
case yet for doing anything with the handler maps other than
passing them off to reader and writer fns.

alexanderkiel/transit-clj,borovsky/transit-clj,cognitect/transit-clj,jdunruh/transit-clj