chunking: Bin packing algorithm which allows to minimize

layer deltas using historical builds

Revamp basic_packing to follow the prior packing structure
if the --prior-build flag exists. This simply modifies existing
layers with upgrades/downgrades/removal of packages. The last layer
contains any new addition to packages.
In the case where --prior-build flag does not exist, the frequency
of updates of the packages (frequencyinfo) and size is utilized to
segment packages into different partitions (all combinations of
low, medium, high frequency and low, medium, high size). The partition
that each package falls into is decided by its deviation from mean.
Then the packages are alloted to different layers to ensure
1) low frequency packages don't mix with high frequency packages
2) High sized packages are alloted separate bins
3) Low sized packages can be put together in the same bin
This problem is aka multi-objective bin packing problem with constraints
aka multiple knapsack problem. The objectives are conflicting given our
constraints and hence a compromise is taken to minimize layer deltas
while respecting the hard limit of overlayfs that the kernel can handle.