changed internal implementation to store all cache types in one file (fixes shared references issue with Storable and largely reduces memory consumption)

fetchrow_arrayref() to speed up DB fetching by approx. 100 rows/sec.

'__END__'...

  Change SQL from 'code like "toplevel"' to 'code = "toplevel"'.

  Formatting

In all the feature adaptors, use either _create_feature() or
_create_feature_fast() from BaseFeatureAdaptor in _objs_from_sth() to
create features.

In get_available_adaptors(), add GeneCollection, TranscriptCollection,
ExonCollection, and RepeatFeatureCollection so that one may say, e.g.

  my $rfc = $registry->get_adaptor( 'Human', 'Core',
                                    'RepeatFeatureCollection' );

Add the methods _create_feature() and _create_feature_fast() that will
be called from _objs_from_sth() in the feature adaptors to create
features.

Turn _remap() into a proper method so that we're able to specialize it.

Beautify.