Implement `TaffyLayout` widget #30

Job	Run time
cargo clippy + test (windows-latest)	9m 15s
cargo fmt	19s
cargo clippy + test (macos-latest)	6m 47s
cargo doc (windows-latest)	2m 13s
cargo doc (macos-latest)	2m 50s
cargo clippy + test (ubuntu-latest)	3m 47s
cargo doc (ubuntu-latest)	2m 7s
	27m 18s

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