Add a `SerializableMap` type #9

Job	Run time
tests (x86_64-unknown-linux-gnu, stable)	29s
codecov	37s
clippy	21s
build (1.81.0, wasm32-unknown-unknown)	24s
rustfmt	11s
build (stable, wasm32-unknown-unknown)	28s
semver	20s
	2m 50s

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