MLDBM - store multi-level hash structure in single level tied hash

This module, intended primarily for use with DBM packages, can serve as a
transparent interface to any TIEHASH package that must be used to
store arbitrary perl data, including nested references.

It works by converting the values in the hash that are references, to their
string representation in perl syntax.  When using a DBM database, it is this
string that gets stored.
