MRG: improve downsampling behavior on `KmerMinHash`; fix `RevIndex::gather` bug around `scaled`. #5810

Job	Run time
Check	29s
test (beta)	1m 31s
test_all_features	14m 0s
test (stable)	1m 37s
test (windows)	4m 19s
test (macos)	1m 46s
Check if cbindgen runs cleanly for generating the C headers	8m 48s
coverage	10m 39s
Lints (beta)	38s
minimum_rust_version	7m 9s
Lints (stable)	28s
Publish (dry-run)	8m 43s
Run tests under wasm32-wasi	1m 12s
Check if wasm-pack builds a valid package for the sourmash crate	1m 19s
	1h 2m 38s

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