feat: `HashBuilder::add_leaf_unchecked` #199

Job	Run time
miri --no-default-features	4m 57s
test stable --no-default-features	20s
test stable	23s
miri	49s
clippy	34s
test stable --all-features	50s
miri --all-features	51s
test beta --no-default-features	37s
test beta	37s
docs	33s
feature-checks	19s
test beta --all-features	50s
test nightly --no-default-features	36s
fmt	12s
test nightly	38s
test nightly --all-features	47s
test 1.79 --no-default-features	13s
test 1.79	13s
test 1.79 --all-features	14s
	14m 33s

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