Add stress tests for all three memory allocators.

[ehpor]

These tests do stress tests on all three memory allocators using N threads and performs M iterations with:

• a chance to allocate memory and store the handle in the buffer
• a chance to deallocate this memory
• a chance to allocate memory permanently
• a chance to move allocated memory to other threads

This tests semi-realistic memory allocation patterns and threading behavior. Tests functions are mostly written in C++ to avoid GIL threading issues hiding potential issues with our memory allocator implementation.

TODO:

• Check whether asserts are strict enough.

Code partially generated using Kimi K2.6.