Consider adding optional headers for abseil

[gerrymanoim]

#include <libpy/abseil_support.h> if you were using abseil, but otherwise unused.

Any other libs to support?

[llllllllll]

It might be worth adding support for some of the Boost containers. I think that we should add a section on the docs showing py::dispatch::map_to_object (and some of the other helpers) to add a to_object adapter to arbitrary mapping types.

[gerrymanoim]

We should also deal with sparsehash, make it optional and add it to a similar _support.h header.

[llllllllll]

Maybe we should have a library_converters dir like: library_converters/abseil.h and library_converters/sparsehash.h.