scripts/buildstats-diff: move code to lib/buildstats.py

scripts/buildstats-diff: move code to lib/buildstats.py

Move over code from buildstats-diff to new scripts/lib/buildstats.py
module in order to share code related to buildstats processing.  Also,
refactor the code, introducing new classes to make the code readable,
maintainable and easier to debug.

[YOCTO #11381]

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>