GDB module to activate frames

Folly's fibers (based on Boost.Context) have a GDB script to debug fibers. The script only has a few helpers to list fibers and inspect a fiber.

Once we find a valid pointer to a `boost::context::fiber` in the debugging session, it'd be useful to have two simple GDB commands to inspect its call stack. The lines related to this feature in the Facebook script are: https://github.com/facebook/folly/blob/85465c8ef64be895bd15a0a315f3698fb5dc685f/folly/fibers/scripts/gdb.py#L191-L247 ([supporting documentation](https://sourceware.org/gdb/current/onlinedocs/gdb/Unwinding-Frames-in-Python.html#Unwinding-Frames-in-Python))