Expose Serde support for `ProofNodes` and `DecodedProofNodes` #255

Job	Run time
feature-checks	18s
clippy	30s
fmt	13s
docs	28s
miri --no-default-features	4m 26s
test stable --no-default-features	22s
miri	4m 44s
miri --all-features	4m 37s
test stable	14s
test stable --all-features	38s
test beta --no-default-features	21s
test beta	24s
test beta --all-features	48s
test nightly --no-default-features	36s
test nightly	37s
test nightly --all-features	46s
test 1.81 --no-default-features	15s
test 1.81	16s
test 1.81 --all-features	27s
deny / cargo deny check	14s
	21m 14s

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