perf: Add more moves and optimize

[Skylion007]

Description

This PR fixes a couple of things:

1. Use member initializers even more aggressively in pytypes.h to remove the need to default initialize m_ptr to null.
2. Apply a bunch of missing std::moves to prevent unecessary pointer increments and decrements
3. Adds a missing a perfect forwarding in a constructor.

Suggested changelog entry:

* Made slice constructor more consistent
* Improve performance of some casters by allowing reference stealing.