Unsafe class discovery

[matyama]

Description

• Adds package unsafe for impure code
• Adds module reflection with functions for discovery of all implementations of a class

Test plan

Passing release check and tested in other project from which is the code being migrated.

[matyama]

Question: Does this code belong to ftoolz?
Answer: Perhaps not, but I don't really want to start and manage new repo for this kind of stuff 🙈

[martinbabka]

Except for pkgutil requirement, looks perfect.

[matyama]

Just found out that there are some issues with the import, I'll investigate but lets postpone merging this one :(

[matyama]

Added a fix which solves the import issues while keeping the discovery API (impementations()) the same.

The only difference is that I moved doctest to ordinary UTs because there were some issues with getting the doctest-defined classes from a module object - some doctest thing I guess.