perf: store RLP-encoded nodes using ArrayVec #156

Job	Run time
docs	24s
clippy	25s
feature-checks	11s
fmt	12s
miri --no-default-features	4m 31s
test stable --no-default-features	14s
miri	4m 32s
test stable	13s
miri --all-features	4m 44s
test stable --all-features	42s
test beta --no-default-features	19s
test beta	17s
test beta --all-features	47s
test nightly --no-default-features	19s
test nightly	20s
test nightly --all-features	48s
test 1.79 --no-default-features	16s
test 1.79	17s
test 1.79 --all-features	27s
	19m 58s

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