Implement `TaffyLayout` widget #14

Job	Run time
cargo fmt	15s
cargo doc (windows-latest)	2m 5s
cargo clippy + test (windows-latest)	1m 43s
cargo doc (macos-latest)	1m 53s
cargo clippy + test (macos-latest)	1m 36s
cargo clippy + test (ubuntu-latest)	1m 59s
cargo doc (ubuntu-latest)	2m 43s
	12m 14s

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