Switch spatial registers to a sparse array

[deft-code]

This is the minimal change needed to use a sparse spatial index.

I think this would be better as a Map (the datatype is more idiomatic for a matrix) but this current change is so small that it's very low risk.

[o4kapuk]

@deft-code what's the point of this change?

[sparr]

This change decreases memory usage and initial allocation time, at the expense of increased runtime for some future array operations.