Add BTreeMapPrefix #322

Job	Run time
missing-files	1s
format	16s
build and test (ubuntu-latest, nightly)	1m 5s
build and test (ubuntu-latest, beta)	58s
build and test (ubuntu-latest, stable)	1m 6s
build and test (macos-latest, nightly)	1m 37s
build and test (macos-latest, beta)	1m 45s
build and test (macos-latest, stable)	2m 27s
build and test (windows-latest, nightly)	2m 56s
build and test (windows-latest, beta)	3m 29s
build and test (windows-latest, stable)	3m 59s
clippy	56s
docs	43s
licenses	2m 13s
udeps	6m 7s
miri (packable)	1m 21s
no_std (nightly, iterator-sorted)	32s
no_std (nightly, prefix-hex)	34s
no_std (beta, iterator-sorted)	28s
no_std (beta, prefix-hex)	29s
no_std (stable, iterator-sorted)	32s
no_std (stable, prefix-hex)	32s
	34m 6s

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