Implement `TaffyLayout` widget #60

Job	Run time
cargo fmt	13s
cargo clippy + test (windows-latest)	2m 5s
cargo doc (windows-latest)	3m 5s
cargo clippy + test (macos-latest)	1m 52s
cargo doc (macos-latest)	1m 41s
cargo clippy + test (ubuntu-latest)	1m 27s
cargo doc (ubuntu-latest)	1m 28s
	11m 51s

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