Split up reducers

[legnes]

This PR splits the giant reducer file into three separate reducers: one for the grid state, one for the builder, and one for the puzzle. It also moves some of the reducer logic into little util files to avoid circularities (which I don't think would break the build or anything, but just to be on the safe side...).

To make the diff easier to read, I copied the old reducer file over to new reducer files and then edited each one down in its own commit (this was @mdirolf's idea).

I think the split-up reducer files should have a few advantages

• More manageable
• Makes it clearer how the different parts of the state are interconnected
• Prerequisite for any kind of future refactoring of the reducer logic or introduction of a state management library

[mdirolf]

I think this is a huge improvement - thanks for putting in all the effort on it!