Implement `TaffyLayout` widget #89

Job	Run time
cargo fmt	11s
cargo doc (windows-latest)	1m 16s
cargo clippy + test (windows-latest)	1m 43s
cargo doc (macos-latest)	49s
cargo clippy + test (macos-latest)	1m 46s
cargo doc (ubuntu-latest)	31s
cargo clippy + test (ubuntu-latest)	1m 0s
	7m 16s

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