Support inverted index keys nicely

[dw]

acid.keylib.packs() currently knows nothing about inverting keys. Some hacks can be used to mostly avoid the necessity, e.g. negating numbers, xorring strings, etc., but this is unintuitive and error prone.

Consider either something like packs(..., invert=...) where invert is some vector map describing which offsets to flip.

Alternatively consider a magical Inverse() wrapper that goes around any primitive value to cause its representation to be flipped.

How should this work for meta.py?