Relax some over-constrained impl bounds #255

Job	Run time
clippy	16s
Check MSRV and minimal-versions	2m 17s
miri	2m 23s
nostd_build (1.63.0, thumbv6m-none-eabi)	2m 5s
tests (1.63.0)	2m 28s
nostd_build (stable, thumbv6m-none-eabi)	18s
tests (stable, arbitrary)	33s
tests (stable, quickcheck)	46s
tests (stable, rayon)	40s
tests (stable, rustc-rayon)	44s
tests (stable, serde)	33s
tests (stable, std)	29s
tests (beta)	36s
tests (nightly, test build benchmarks)	37s
	14m 45s

Provide feedback