Add a `SerializableMap` type #8

Job	Run time
rustfmt	12s
codecov	36s
build (1.81.0, wasm32-unknown-unknown)	25s
clippy	22s
build (stable, wasm32-unknown-unknown)	25s
semver	2m 34s
tests (x86_64-unknown-linux-gnu, stable)	29s
	5m 3s

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