Implement `TaffyLayout` widget #95

Job	Run time
cargo fmt	11s
cargo doc (windows-latest)	1m 4s
cargo clippy + test (windows-latest)	1m 57s
cargo doc (macos-latest)	47s
cargo clippy + test (macos-latest)	1m 45s
cargo doc (ubuntu-latest)	34s
cargo clippy + test (ubuntu-latest)	1m 21s
	7m 39s

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