feat!: Update serialisation schema, implement `CustomConst` serialisation #404

Job	Run time
Check for changes in Rust files	4s
tests (Rust 1.75)	1m 13s
check	58s
tests (Rust beta)	1m 12s
benches	1m 18s
tests (Rust stable)	1m 16s
tests (Rust nightly)	1m 28s
Required checks 🦀	1s
coverage	2m 21s
	9m 51s

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