Circuit oram/adding linearity test

[wjuan-mob]

Adding measure_oram_stash_size_distribution function that returns stash size statistics
Adding tests for stash size independence with respect to oram size
Adding tests for stash size linear growth with respect to time.
Increases the size and timeout of tests in CircleCI to run these since otherwise it times out and runs out of memory.

These tests are generally useful for determining whether our Oram implementations are well behaved, and helps validate changes to our oram.

These tests are mimicking figures 3 and 4 in this paper: https://eprint.iacr.org/2013/280.pdf
The tolerances for the unit tests were increased far above what may appear normal in order to keep the CI runs under an hour. In order to achieve tolerance under .05 percent, it was necessary to increase the number of iterations over 2^25 which takes between 5 and 10 hours depending on the machine etc. Though optimizations could be made to parallelize runs if this is desired.
The choice to use variance instead of using the thresholding scheme used in the paper to test independence against N is because it seemed like a more intuitive metric for the unit test that would be more granular/sensitive to changes.

[samdealy]

Nice work, thanks for meeting to go more in depth!