Trace hooks as part of a session trace

[siddharthab]

This allows to instrument packages such that a single test run can trace multiple packages and output trace counters for all of them together on session exit.

In my understanding, without this, the counters would reset whenever a new instrumented package is loaded while the tests are running.

I am not sure if this is the best design, but wanted to start a discussion on how best to profile multiple packages through a single test suite.