feat: impl Extend for ProofNodes #195

Job	Run time
docs	32s
clippy	34s
feature-checks	51s
fmt	14s
miri --no-default-features	4m 35s
miri	49s
test stable --no-default-features	25s
miri --all-features	50s
test stable	25s
test stable --all-features	45s
test beta --no-default-features	35s
test beta	36s
test beta --all-features	49s
test nightly --no-default-features	35s
test nightly	34s
test nightly --all-features	45s
test 1.79 --no-default-features	18s
test 1.79	17s
test 1.79 --all-features	26s
	14m 55s

Provide feedback