Make `Round` dyn-safe #222

Job	Run time
rustfmt	12s
clippy	27s
build-docs	36s
semver	2m 36s
stable-test (x86_64-unknown-linux-gnu, 1.81.0)	43s
test-and-coverage	49s
wasm-build (1.81.0, wasm32-unknown-unknown)	25s
wasm-build (stable, wasm32-unknown-unknown)	26s
	6m 14s

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