Convert song classes into either Pydantic `BaseModels` or `dataclasses`

[vivaria]

• https://docs.pydantic.dev/latest/
• https://medium.com/towards-data-science/battle-of-the-data-containers-which-python-typed-structure-is-the-best-6d28fde824e

I've implemented Pydantic support in viv/test-out-pydantic, and it's uncovered a few bugs (yay!), but it incurs a fairly big performance hit for type validation. I don't really want to add such a slow down to production (it's pretty antithetical to #55), so I expect that I'll eventually settle on dataclasses instead? :thinking:

There's a way to implement type conversion for dataclasses here: https://stackoverflow.com/a/54863733