This class implements an ordered hash-like object. It's a cross
between a Perl hash and a linked list. Use it whenever you want the
speed and structure of a Perl hash, but the orderedness of a list.

Don't use it if you want to be able to address your hash entries
by number, like you can in a real list ($list[5]).

See also Tie::IxHash by Gurusamy Sarathy. It's similar (it also
does ordered hashes), but it has a different internal data structure
and a different flavor of usage. IxHash stores its data internally
as both a hash and an array in parallel. LLHash stores its data as
a bidirectional linked list, making both inserts and deletes very
fast. IxHash therefore makes your hash behave more like a list than
LLHash does. This module keeps more of the hash flavor.
