Add BTreeMapPrefix #324

Job	Run time
build and test (ubuntu-latest, nightly)	1m 11s
missing-files	5s
build and test (ubuntu-latest, beta)	1m 3s
format	11s
build and test (ubuntu-latest, stable)	1m 0s
build and test (macos-latest, nightly)	2m 47s
build and test (macos-latest, beta)	3m 24s
build and test (macos-latest, stable)	2m 44s
build and test (windows-latest, nightly)	3m 12s
build and test (windows-latest, beta)	2m 21s
build and test (windows-latest, stable)	2m 49s
clippy	46s
docs	42s
udeps	6m 5s
licenses	2m 0s
miri (packable)	1m 32s
no_std (nightly, iterator-sorted)	35s
no_std (nightly, prefix-hex)	27s
no_std (beta, iterator-sorted)	26s
no_std (beta, prefix-hex)	26s
no_std (stable, iterator-sorted)	38s
no_std (stable, prefix-hex)	30s
	34m 54s

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