Implement `TaffyLayout` widget #41

Job	Run time
cargo fmt	16s
cargo doc (windows-latest)	2m 40s
cargo clippy + test (windows-latest)	2m 40s
cargo doc (macos-latest)	2m 25s
cargo clippy + test (macos-latest)	1m 42s
cargo doc (ubuntu-latest)	1m 32s
cargo clippy + test (ubuntu-latest)	1m 49s
	13m 4s

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