Circuit padding trace simulator

This is the parent ticket for the pieces of work required to make it possible to use circuitpadding.c in a trace simulator outside of Tor, so that defenses could be re-applied to crawl traces quickly without needing to re-crawl a set of sites.

An alternate way to do this, instead of extracting this code, is to make use of our unit testing framework and build the tracer as a unit test. We have mechanisms to mock the networking functions so that they output new traces, and then we can use the unit-test style to read in a trace file and output a new one, instead of performing a test.