feat: ssz encode and decode for uint #779

Job	Run time
Coverage	3m 32s
Cargo deny check	28s
Docs	1m 8s
Clippy	1m 12s
Feature checks	23s
Fmt	16s
test (stable, --no-default-features)	2m 36s
test (stable)	1m 22s
test (stable, --all-features)	4m 8s
test (beta, --no-default-features)	3m 34s
test (beta)	2m 51s
test (beta, --all-features)	2m 47s
test (nightly, --no-default-features)	2m 3s
test (nightly)	3m 21s
test (nightly, --all-features)	3m 52s
test (1.65, --no-default-features)	35s
test (1.65)	18s
Accept	1s
	34m 27s

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