Implement `TaffyLayout` widget #19

Job	Run time
cargo fmt	18s
cargo doc (windows-latest)	4m 28s
cargo clippy + test (windows-latest)	3m 18s
cargo doc (macos-latest)	1m 42s
cargo clippy + test (macos-latest)	2m 6s
cargo doc (ubuntu-latest)	2m 30s
cargo clippy + test (ubuntu-latest)	2m 31s
	16m 53s

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