Move cpp_function to its own header file for further refactoring

[lqf96]

Description

Move cpp_function to its own header file pybind11/detail/function.h, because I find it hard to refactor cpp_function as pybind11/pybind11.h is too long. I specifically choose the detail directory as I'd like to split cpp_function into two parts in the long term: one part (in pybind11 namespace) being an object wrapper around a C++ function and another part (in detail namespace) dispatching the actual function call. I'll provide more details about these refactoring in upcoming PRs. This PR also moves signature and docstring generation of cpp_function to separate functions.

[bstaletic]

Any reason this was changed from pybind11::repr to repr? It should be perfectly fine, since args_[ti] has a type from the pybind11 namespace, so even ADL would only find pybind11::repr, just curious if the change was intentional.

[YannickJadoul]

I'd like to first get some context on this refactoring, before moving things around. Can you please create an issue with what you're planning to do and, more importantly, why, so that we can discuss? Anything that's refactoring something this big should be discussed with maintainers, before just throwing a series of PRs at us, since it has implications for existing PRs and forks, and shouldn't just be done "for fun".

Take the interaction with #2445 (first proposed/discussed in #2322) as an example of why this needs to be coordinated.

[lqf96]

@YannickJadoul Yeah I agree we should probably discuss about this in advance... Let me open an issue later about the motivations and intentions...

[lqf96]

@bstaletic @YannickJadoul I just opened #2810 to describe the problem I ran into, the refactors I want to do and the potential problems. It is more like a RFC so feel free to give me suggestions there.

[YannickJadoul]

@lqf96, thanks, I saw! When I (soon) pull together my courage to think about this, I'll have a closer look and comment in that PR :-)