Add method to get a map entry by index #244

Job	Run time
clippy	12s
Check MSRV and minimal-versions	1m 35s
nostd_build (1.63.0, thumbv6m-none-eabi)	1m 19s
miri	2m 21s
nostd_build (stable, thumbv6m-none-eabi)	15s
tests (1.63.0)	1m 50s
tests (stable, arbitrary)	36s
tests (stable, quickcheck)	36s
tests (stable, rayon)	44s
tests (stable, rustc-rayon)	41s
tests (stable, serde)	39s
tests (stable, std)	37s
tests (beta)	37s
tests (nightly, test build benchmarks)	38s
	12m 40s

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