feat!: Update serialisation schema, implement `CustomConst` serialisation #439

Job	Run time
Check for changes in Rust files	10s
check	43s
benches	1m 47s
tests (Rust 1.75)	1m 26s
tests (Rust stable)	1m 21s
tests (Rust beta)	1m 18s
tests (Rust nightly)	1m 22s
Required checks 🦀	1s
coverage	2m 36s
	10m 44s

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