feat: add support for more RNTuple types

[ariostas]

This PR adds support for a couple of RNTuple types that are currently not supported, namely std::atomic<T> and std::bitset. I'll make a PR to scikit-hep-testdata to add those, and then set up some tests here.

[ariostas]

There's two more fixes that I'll add to this PR. Currently, if you have an std::variant where one of the types is an empty struct, something goes wrong and it ends up discarding that part of the variant. The other one, is that if you try to read a variant in an invalid state it just doesn't work.