Merge in-progress Square-1 code

[lgarron]

I'm about to start porting GenericPuzzle and related code from phase2-symmcoords, and coordinating two branches is likely to be very messy. It's simpler to merge the Square-1 code, which is stable apart from the Square-1 specific changes. Those changes have been put behind a env var. To test Square-1:

env EXPERIMENTAL_SQUARE1=true cargo run --release -- scramble sq1