Introduce def_visitor abstraction for objects that provide custom binding logic when passed to def()

[oremanj]

This has two purposes:

• I'm planning to contribute some class_ extensions (for pickling and buffer protocol support) that I would like to be able to put in their own headers so they don't slow down compilation for those that don't use them. This PR allows class_::def() to perform an operation that was not foreseen by the code inside class_.
• I've found it quite ergonomically useful (and am carrying a local patch) to be able to add customizations by chaining .def(my_thing()) to a class binding, rather than the existing supported alternative of giving the binding a name and calling a helper function on it (auto someCls = nb::class_<...>(...); my_thing(someCls);).

Previously class_filler, renamed to def_visitor per feedback.

[wjakob]

This is great! I like def_visitor as name.

[oremanj]

Great, renamed!

[wjakob]

Thanks!

[ManifoldFR]

Wow, thanks for adding this!!
This should make the transition for some of us who still use Boost.Python (and hadn't moved to Pybind11) much easier, since BP also had this def_visitor method-chaining pattern.