Add _PyFrame_NumSlotsForCodeObject()

[iritkatriel]

The calculation of how many slots need to be in the frame of a given code object is repeated in a number of places in the codebase, which makes it error prone to try and change the frame layout. It should be refactored into a single place.

(lessons learnt from the register machine experiment).

Linked PRs

• gh-100720: refactor calculation of number of frame slots for a code object into the new function _PyFrame_NumSlotsForCodeObject #100722