Redesign zoned layout statistics

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.