chore(hash-builder): use `RlpNode::as_hash` #181

Job	Run time
fmt	11s
docs	14s
feature-checks	47s
clippy	15s
miri --no-default-features	4m 39s
test stable --no-default-features	31s
miri	4m 35s
miri --all-features	4m 45s
test stable	28s
test stable --all-features	45s
test beta --no-default-features	25s
test beta	25s
test beta --all-features	49s
test nightly --no-default-features	23s
test nightly	22s
test nightly --all-features	46s
test 1.79 --no-default-features	24s
test 1.79	23s
test 1.79 --all-features	32s
	21m 39s

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