This patch improves the builtin dir() function by probing the target object
with all possible qstrs via mp_load_method_maybe.  This is very simple (in
terms of implementation), doesn't require recursion, and allows to list all
methods of user-defined classes (without duplicates) even if they have
multiple inheritance with a common parent.  The downside is that it can be
slow because it has to iterate through all the qstrs in the system, but
the "dir()" function is anyway mostly used for testing frameworks and user
introspection of types, so speed is not considered a priority.

In addition to providing a more complete implementation of dir(), this
patch is simpler than the previous implementation and saves some code
space:

   bare-arm:   -80
minimal x86:   -80
   unix x64:   -56
unix nanbox:   -48
      stm32:   -80
     cc3200:   -80
    esp8266:  -104
      esp32:   -64