Documentation patch: Tie::Hash, Tie::Scalar, Tie::SubstrHash

Documentation patch: Tie::Hash, Tie::Scalar, Tie::SubstrHash

[This finishes a job started by Randy Ray with perl5.002b1f.]

This patch updates the files lib/TieHash.pm and lib/SubstrHash.pm. Those
two files are moved to a new directory called lib/Tie. A new tie-related
class for scalars, lib/Tie/Scalar.pm, is added. Each of these has pod
documentation that passes through pod2man without complaint. All package
names are updated to reflect this structure, and the "standard" sub-classes
for hashes and scalars are named Tie::StdHash and Tie::StdScalar, in
keeping with the group consensus.

With this, the documentation for Tie::Hash should be fine, Tie::Scalar
exists and is documented, and Tie::SubstrHash (formerly just SubstrHash)
is also documented, albeit weakly. I have no examples of usage to show, and
I suspect a possible bug or two, but without examples it's hard to say.

As it happens, this patch also impacts the DBM-style extensions, as they
(DB_File, NDBM_File, GBM_File, ODBM_File, SDBM_File) all inherited from
the old TieHash.pm as a base class.