Modules

I think compartmentalizing Html, Props, and Prop away from the main Shpadoinkle.Core module would be a good decision. It makes the codebase a bit more readable, and could allow for easier pop-and-swaps in the future (maybe something like TextMap for a GHC-only version of Props, or something similar.

Furthermore, a lot of the constraints in many terms relied on Monad m =>, when Applicative m => or even Functor m => would suffice. This may not realistically improve performance, but still is generally a good habit IMHO.