Implement `TaffyLayout` widget #40

Job	Run time
cargo clippy + test (windows-latest)	2m 43s
cargo doc (windows-latest)	3m 9s
cargo fmt	12s
cargo clippy + test (macos-latest)	1m 22s
cargo doc (macos-latest)	1m 53s
cargo clippy + test (ubuntu-latest)	1m 0s
cargo doc (ubuntu-latest)	3m 10s
	13m 29s

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