core: remove suffix n in serde `u64` serialization & deserialization #512

Job	Run time
Dusk Analyzer / Dusk Analyzer	20s
Code Analysis / Clippy Check	27s
test cirucits / Run tests	3m 12s
Code Analysis / Rust Format	19s
test core / Run tests	27s
test core rkyv compiles / Run tests	12s
test cirucits no-default-features / Run tests	29s
	5m 26s

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