feat!: Update serialisation schema, implement `CustomConst` serialisation #371

Job	Run time
Check for changes in Rust files	4s
check	1m 5s
benches	1m 54s
tests (Rust 1.75)	1m 16s
tests (Rust stable)	1m 20s
tests (Rust beta)	1m 26s
tests (Rust nightly)	1m 47s
Required checks 🦀	1s
coverage	4m 1s
	12m 54s

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