Separate the public & private scope from the existing "no scope" mechanism

[guruappa]

Separate functions scope into public and private scope: public functions are the ones the users will call, vs private functions are for internal use.

_single_leading_underscore: weak “internal use” indicator.

E.g. from M import * does not import objects whose names start with an underscore.

[guruappa]

This is done, by changing the codebase from function base to OOP paradigm