Implement `TaffyLayout` widget #55

Job	Run time
cargo doc (windows-latest)	1m 56s
cargo fmt	13s
cargo doc (macos-latest)	40s
cargo clippy + test (windows-latest)	2m 8s
cargo doc (ubuntu-latest)	33s
cargo clippy + test (macos-latest)	1m 5s
cargo clippy + test (ubuntu-latest)	1m 3s
	7m 38s

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