feat: Stubs and RowGroups now inherit from Sequence

[machow]

This PR fleshes out the Stub and RowGroups classes, so they implement collections.abc.Sequence. This allows them to behave like (immutable) lists.

It also makes RowInfo a dataclass, which simplifies its construction a bit, and lets us do equality checks on them.

See https://docs.python.org/3/library/collections.abc.html for more details.

TODO: I'm going to try scaffolding out a few functions that use these classes to be sure they're working okay!

[machow]

@rich-iannone I think this might be ready for you to review! It's a doozy, but I'm trying to shift the classes away from SomeClass([1,2,3])._some_class[1] to being used directly (e.g. SomeClass([1,2,3])[1]).

I tried to implement 3 functions:

• body_rearrange
• reorder_stub_df
• get_row_reorder_df

which required fleshing out 4 classes: Stub, RowGroups, Boxhead, Body

I'm not totally sure exactly what they do, and just tried to follow the R code. So might need to lean on you for testing them 🙈

[rich-iannone]

Lgtm!