Automatically detect memory use regressions

[samcrow]

From #55 I learned that people are using canadensis on microcontrollers with very limited memory. Release 0.5.0 made canadensis_can use more memory, but I didn't know at first that the change would be a problem.

To reduce the risk of that happening again, we might be able to set up a test that runs some representative workload, measures memory use at various times, and fails or gives a warning if the memory use is too high.