Add empty() method to set/dict/tuple/list/mapping for a parity with pybind11

[vfdev-5]

Hi @wjakob , I would like to see if you will be supportive for adding empty() method to structures like set/dict/list/mapping/tuple/list to have a certain degree of parity with pybind11. For context, recently I ported a code from pybind11 to nanobind and was expecting to have a similar code to if (kwargs.empty()) {, but seems like nanobind does not have these methods. What do you think about adding these methods?
I coded that here: https://github.com/wjakob/nanobind/compare/master...vfdev-5:nanobind:dict-empty-method?expand=1
If you are OK to add them, I can send it as a PR.
Thanks

[wjakob]

I am open to adding this. Please go ahead and make a PR (including API docs updates).