Add `just`: new constructor to `IndexSet` and `IndexMap` respectively #353

Job	Run time
miri	3m 30s
Check MSRV and minimal-versions	2m 3s
clippy	15s
nostd_build (1.63.0, thumbv6m-none-eabi)	1m 49s
nostd_build (stable, thumbv6m-none-eabi)	10s
tests (1.63.0)	2m 13s
tests (stable, arbitrary)	39s
tests (stable, quickcheck)	41s
tests (stable, rayon)	50s
tests (stable, rustc-rayon)	49s
tests (stable, serde)	35s
tests (stable, borsh)	30s
tests (stable, std)	45s
tests (beta)	34s
tests (nightly, test build benchmarks)	36s
Success	1s
	16m 0s

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