Support for NamedTuple

[jdumas]

Unless I missed something, I haven't seen in the doc/github any mention of nanobind supporting NamedTuple. It would be nice to have some helpers to facilitate the creation of named tuple objects e.g. instead of simply returning a plain tuple as the result of a function.

[wjakob]

I am open to PRs in that direction but have not planned to implement this myself.