loadelement.py: Use roaring bitmaps in dep cache

The dependency cache in LoadElement causes the peak RAM consumption of
the loader to be exceedingly large, upwards of 25GB for a test of a
Debian stack.  By switching from the old dict cache to a roaring bitmap
based cache, we reduce the cost of the loader cache in RAM terms to
around 5.5GB for the same stack, which makes it plausible for the short
term.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>