MRG: refactor `calculate_gather_stats` to disallow repeated downsampling #5812

Job	Run time
Check	34s
coverage	11m 5s
Lints (beta)	32s
Lints (stable)	37s
Publish (dry-run)	6m 11s
test (beta)	1m 32s
test (stable)	1m 34s
test_all_features	13m 47s
test (windows)	3m 45s
test (macos)	1m 21s
Check if wasm-pack builds a valid package for the sourmash crate	1m 22s
Run tests under wasm32-wasi	1m 14s
Check if cbindgen runs cleanly for generating the C headers	8m 46s
minimum_rust_version	7m 33s
	59m 53s

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