Add new Unwind API. This is the final envisioned API with the correct
abstraction level. It hides/slow fast unwinder selection from the caller
and doesn't take any arguments that would leak that abstraction (i.e.,
arguments like stack_top/stack_bottom).
GetStackTrace will become an implementation detail (private method) of
the BufferedStackTrace class.
Let's postpone cleaning up the individual GetStackTrace implementations
until the surrounding code is in its final state.
do you need fast = false; here?