Use `OctantMap` in `Alloctree`. #612

Job	Run time
deny	44s
miri	1m 58s
build (ubuntu, stable, locked, true)	12m 52s
incremental-mutants	0s
build (windows, stable, locked, false)	25m 10s
no_std	42s
build (macos, stable, locked, false)	11m 43s
build (ubuntu, nightly, locked, false)	26m 6s
build (ubuntu, beta, locked, false)	23m 3s
build (ubuntu, stable, latest, false)	14m 59s
build (ubuntu, stable, minimal, false)	14m 37s
features	29m 38s
fuzz	15m 45s
deploy	8s
	2h 57m 25s

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