Use usize indexes

[Ten0]

This allows for unioning bigger sets if necessary, but also doesn't necessarily change the storage size because it may still downsize to an u32 in from_index.

This shouldn't change anything to perf if the caller still uses u32 in their UnifyKey, considering that converting from usize to u32 was already done just before these function calls, and now it will be done just after, and they will likely be inlined.

The only place I can see where this may have implications is on rank, which may need to be made bigger as well... but it looks like the max rank is actually logarithmic in the number of nodes if we merge favoring min rank so it's unlikely to be necessary.