Add `VoxelOpacityMask` type for `EvaluatedBlock::voxel_opacity_mask()`. #688

Job	Run time
deny	45s
incremental-mutants	0s
build (ubuntu, stable, locked, true)	11m 41s
lint (stable)	7m 47s
build (windows, stable, locked, false)	21m 6s
miri	1m 52s
no_std	53s
lint (nightly)	7m 27s
build (macos, stable, locked, false)	8m 59s
lint (beta)	7m 49s
build (ubuntu, nightly, locked, false)	23m 14s
build (ubuntu, beta, locked, false)	21m 26s
build (ubuntu, stable, latest, false)	10m 35s
build (ubuntu, stable, minimal, false)	11m 3s
features	30m 17s
fuzz	15m 16s
deploy	5s
	3h 0m 15s

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