Redesign chunked layout statistics

[gatesn]

We should make it such that the stats table stores a mirror of the DType underneath each stats column. For example, min: i32 for an i32 array; or min: {a: i32, b: i32} for a struct array of type {a: i32, b: i32}.

This effectively means a chunked array can store stats for all flattened nested fields.