Script to compare two runlim runs

[aseemr]

A wrapper over existing runlim_stats.py:

• Adds a way to optionally specify an identifier with a runlim invocation. For example, one can say:

make -C ulib/ MON=1 MONID=1

• Adds a script that can compare two runlim runs and plot regression slope, e.g.

$ make -C ulib/ MON=1 MONID=1
$ ... // some work, e.g., local changes or even git pull another branch
$ make -C ulib/ MON=1 MONID=2
$ python3 runlim_diff.py 2 1

Comment from the header of runlim_diff.py:

# This script compares two runlim runs and produces a scatter plot with linear regression
#
# It takes as input two identifiers, the identifiers for the new run and the old run resp.
#
# Suppose the requirement is to compare the runlim runs across two branches b1 and b2
# First you would git checkout b1, and run F* with MON=1 and MONID=1 (or any other identifier)
# Then you would git checkout b2, and run F* with MON=1 and MONID=2 (or something else)
# And then you can invoke this script as `python3 runlim_diff.py 2 1`,
#   and it would compare the run 2 with the baseline run 1
#
# It can be used across local changes also, not just to compare two branches