TODO

- make form state ids sequential and unique
- guard form state ids from tampering by using an HMAC of some sort




The following options may eventually be added to C<init> if there is
demand:

=over 4

=item digest_module

Which digest module to use to create the checksum portion of the session
key.  By default this is C<Digest::MD5>, but you could use any other
module with the same interface, such as C<Digest::SHA1>.

=item keyname_generator

Normally the form state is stored in the user's session
session using a key like:

   form_state_storage_name_84eb13cfed01764d9c401219faa56d53

If you want to name keys differently, you can provide a subroutine to
generate the name.  The subroutine is passed the name of the form state
storage.

=back