feat!: Update serialisation schema, implement `CustomConst` serialisation #422

Job	Run time
Check for changes in Rust files	3s
tests (Rust stable)	1m 35s
check	46s
tests (Rust 1.75)	1m 20s
benches	1m 39s
tests (Rust beta)	1m 24s
tests (Rust nightly)	1m 32s
Required checks 🦀	7s
coverage	0s
	8m 26s

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