Implement `TaffyLayout` widget #13

Job	Run time
cargo clippy + test (windows-latest)	1m 30s
cargo fmt	20s
cargo doc (windows-latest)	4m 36s
cargo clippy + test (macos-latest)	1m 26s
cargo doc (macos-latest)	3m 6s
cargo clippy + test (ubuntu-latest)	1m 43s
cargo doc (ubuntu-latest)	2m 21s
	15m 2s

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