equihash: Fix some Rust API issues #4312

Job	Run time
Build target wasm32-wasi	2m 4s
Latest build on ubuntu-latest	1m 58s
Latest build on windows-latest	3m 25s
Bitrot check	3m 9s
Clippy (beta)	16s
Latest build on macOS-latest	4m 24s
Code coverage	17m 45s
Rustfmt	23s
Test on ubuntu-latest	11m 15s
Intra-doc links	3m 48s
UUID validity	5s
Test on windows-latest	15m 54s
Test on macOS-latest	20m 6s
Clippy (MSRV)	3m 24s
	1h 27m 56s

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