feat: retain hash masks of branch nodes #210

Job	Run time
docs	17s
clippy	21s
feature-checks	29s
fmt	10s
miri --no-default-features	4m 36s
miri	36s
test stable --no-default-features	23s
miri --all-features	54s
test stable	11s
test stable --all-features	35s
test beta --no-default-features	18s
test beta	20s
test beta --all-features	46s
test nightly --no-default-features	24s
test nightly	27s
test nightly --all-features	46s
test 1.79 --no-default-features	18s
test 1.79	14s
test 1.79 --all-features	16s
	12m 21s

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