Refactor(Optimizer): improve typing coverage of optimizer modules

[OutSquareCapital]

Details

WIP

This is a work in progress. But since those files are very untyped ATM, it's better to split up in various passes. This is the first one, focusing on public level and simplify module.

datetime import

I changed the import datetime to from datetime import x, y, z. It's more concise when the datetime object are needed, but most importantly it's much less confusing.
Fault is on Python for sure for the API design, but since datetime is both a module and an object, and datetime.date can represent both a method and an object, IMO it's preferrable.

Partial typing

I skipped various methods/functions (or partially typed them like simplify_literals).
This is intentionnal. ATM there's various untypable behaviors, like "unsafe" non-none checks on Expr properties/args.
This would require changing the actual runtime behavior, which is out-of-scope here.

[OutSquareCapital]

To clarify: the goal is to gradually add types, since optimizers as a whole is a lot more untyped than the rest of the codebase.
As such, this PR scope is:

• fully type the public API (exported via __init__)
• Fully type "easy" functions in the simplify module
• Partially annotate complex blocks with hard-to-type structure like _simplify_comparison in the simplify module

The latter will be revisited in the future, hopefully with the help of more context.
This is precisely why object is a good fit most of the time: mypy don't check the body of untyped functions, and as such we can expect to be more precise as the typing coverage progress and mypy flag object has no attribute "x" problems