Implement `TaffyLayout` widget #17

Job	Run time
cargo fmt	19s
cargo doc (windows-latest)	1m 43s
cargo clippy + test (windows-latest)	2m 27s
cargo doc (macos-latest)	1m 16s
cargo clippy + test (macos-latest)	2m 18s
cargo doc (ubuntu-latest)	43s
cargo clippy + test (ubuntu-latest)	1m 52s
	10m 38s

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