Implement `TaffyLayout` widget #18

Job	Run time
cargo fmt	13s
cargo doc (windows-latest)	1m 30s
cargo clippy + test (windows-latest)	3m 5s
cargo doc (macos-latest)	36s
cargo clippy + test (macos-latest)	1m 36s
cargo doc (ubuntu-latest)	49s
cargo clippy + test (ubuntu-latest)	1m 59s
	9m 48s

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