Support resizing Grids / Refactor away const generics

[ecton]

Const generics for the number of "columns" in a grid was a neat experiment, but it's not really practical. When working on the goosheet (I need to come up with a new name, and I'm not sure I love cusheet 😆), I realized I needed the ability to add and remove columns. Const generics aren't really there for being able to do this type of operation.

Instead, We should remove the const generic and just handle mistmatches in child count + cell count by just treating the remaining cells as empty.