MRG: refactor `calculate_gather_stats` to disallow repeated downsampl… #5813

Job	Run time
Check	29s
coverage	10m 19s
Lints (beta)	32s
Lints (stable)	27s
minimum_rust_version	7m 10s
Publish (dry-run)	6m 22s
test (beta)	1m 32s
test_all_features	13m 57s
test (stable)	1m 32s
Run tests under wasm32-wasi	1m 13s
Check if wasm-pack builds a valid package for the sourmash crate	1m 34s
test (windows)	4m 46s
Check if cbindgen runs cleanly for generating the C headers	8m 21s
test (macos)	1m 24s
	59m 38s

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