Implement `TaffyLayout` widget #15

Job	Run time
cargo fmt	16s
cargo clippy + test (windows-latest)	8m 46s
cargo doc (windows-latest)	2m 12s
cargo clippy + test (macos-latest)	4m 29s
cargo doc (macos-latest)	54s
cargo clippy + test (ubuntu-latest)	3m 41s
cargo doc (ubuntu-latest)	42s
	21m 0s

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