Add datetime wrapper class

[WillAyd]

Per discussion in #394

I couldn't find a test suite for the similarly designed classes so not sure how to best assert the functionality here. Open to any pointers you might have

[wjakob]

nanobind core header files absolutely cannot depend on large headers like <chrono>, which seems fundamental to this PR.

The rationale is explained in the documentation:

Smaller headers: #include <pybind11/pybind11.h> pulls in a large portion of the STL (about 2.1 MiB of headers with Clang and libc++). nanobind minimizes STL usage to avoid this problem. Type casters even for for basic types like std::string require an explicit opt-in by including an extra header file (e.g. #include <nanobind/stl/string.h>).

[WillAyd]

Ah ok thanks for that context. As an alternative would you consider a custom struct that is like struct tm but with microsecond support?

That would avoid the stl import issues but still give users some way to create a datetime object directly

[wjakob]

I would say that there isn't an obvious or elegant way to expose this particular type as a wrapper, compared to most other ones. Since there isn't really any harm in adding extra wrappers in user projects, I'd say that this can exist downstream outside of nanobind.