tor/changes/fancy_testing

  o Build features:

    - Tor now builds each source file in two modes: a mode that avoids
      exposing identifiers needlessly, and another mode that exposes
      more identifiers for testing. This lets the compiler do better at
      optimizing the production code, while enabling us to take more
      radical measures to let the unit tests test things.

    - The production builds no longer include functions used only
      in the unit tests; all functions exposed from a module for
      unit-testing only are now static in production builds.

    - Add an --enable-coverage configuration option to make the unit
      tests (and a new src/or/tor-cov target) to build with gcov test
      coverage support.

  o Testing:

    - We now have rudimentary function mocking support that our unit
      tests can use to test functions in isolation. Function mocking
      lets the tests temporarily replace a function's dependencies with
      stub functions, so that the tests can check the function without
      invoking the other functions it calls.

    - Add more unit tests for the <circid,channel>->circuit map, and
      the destroy-cell-tracking code to fix bug 7912.