* in the aw_backtrace callback API consider extending pc_before_insn to
  flags. To support AW_FRAME_IS_SIGNAL indicator too.

* make the code clean on -Wconversion. Painful but worth it. Core of
  backtracer is mostly done, but needs to be permanent and do the
  rest. Introduce relevant helpers to make it nice and readable.

* signal frame should have callback invoked with is_leaf = true
  (because it is "exact pc"). Actually double check
  this. libgcc/libbacktrace don't do it. So we need to decide.

* consider exposing some richer indication whether backtrace captured
  was reliable/full or if we truncated due to error. And perhaps which
  frames we had to guess.

