MRG: add public `n_hashes` fn to signature #6218

Job	Run time
Check	26s
coverage	7m 30s
minimum_rust_version	6m 38s
Publish (on tags, dry-run otherwise)	5m 15s
Lints (beta)	35s
Lints (stable)	25s
test (beta)	1m 5s
test_all_features	4m 6s
test (stable)	1m 1s
Run tests under wasm32-wasi	1m 11s
test (windows)	3m 15s
test (macos)	59s
Check if wasm-pack builds a valid package for the sourmash crate	1m 15s
Check if cbindgen runs cleanly for generating the C headers	8m 5s
	41m 46s

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