Implement `TaffyLayout` widget #51

Job	Run time
cargo fmt	13s
cargo doc (windows-latest)	3m 18s
cargo clippy + test (windows-latest)	2m 16s
cargo doc (macos-latest)	2m 27s
cargo clippy + test (macos-latest)	1m 53s
cargo doc (ubuntu-latest)	1m 37s
cargo clippy + test (ubuntu-latest)	1m 24s
	13m 8s

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