MRG: refactor `calculate_gather_stats` to disallow repeated downsampling #5805

Job	Run time
Check	25s
coverage	10m 18s
Check if cbindgen runs cleanly for generating the C headers	8m 40s
Lints (beta)	34s
Lints (stable)	28s
minimum_rust_version	7m 8s
Publish (dry-run)	6m 8s
test_all_features	13m 47s
test (beta)	1m 32s
Run tests under wasm32-wasi	1m 23s
Check if wasm-pack builds a valid package for the sourmash crate	1m 25s
test (stable)	1m 31s
test (windows)	3m 32s
test (macos)	1m 34s
	58m 25s

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