Add `multiset_permutations` iterator #1054

Job	Run time
msrv	29s
check	11s
test	50s
check (--no-default-features)	8s
doc	6s
check (--no-default-features --features use_alloc)	10s
check format	7s
check (--all-targets --all-features)	12s
miri	3m 29s
semver-checks	17s
All checks succeeded	0s
	5m 59s

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