Generalize `GridArray` into `Vol` which allows borrowed data. #121

Job	Run time
miri	1m 50s
build (ubuntu, stable, locked, true)	51m 30s
build (windows, stable, locked, false)	23m 21s
build (macos, stable, locked, false)	20m 36s
build (ubuntu, nightly, locked, false)	28m 58s
build (ubuntu, beta, locked, false)	14m 36s
build (ubuntu, stable, latest, false)	24m 32s
build (ubuntu, stable, minimal, false)	24m 33s
deploy	9s
fuzz	17m 10s
	3h 27m 15s

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