Cast member pointer arguments of class_<T>::def() to member-of-T

[oremanj]

Fixes #470; see that issue for a more detailed explanation of the problem. This ports over pybind's solution with some C++ modernization.

[wjakob]

I have been negative about this particular change in the past because I felt it adds too much boilerplate + templates that have to be instantiated for every function binding. I am not totally opposed but mainly interested in finding a simple and efficient solution.

[oremanj]

Thanks for encouraging me to simplify this. Please take another look; I think you'll be much happier with the second version. It should work on anything that cpp_function does, because it's implemented inside the relevant overloads of cpp_function.

[wjakob]

Yes, I am much happier with this version, thank you for making the changes. This looks good to merge to me, shall I?

[oremanj]

Go for it - thank you!

[wjakob]

Sorry, one more request: could you add a changelog entry?

[wjakob]

I think that the issue in #284 is also simplified by this commit.

[oremanj]

Added changelog entry.

[wjakob]

Thank you 👍