Implement `TaffyLayout` widget #78

Job	Run time
cargo fmt	10s
cargo doc (windows-latest)	2m 55s
cargo clippy + test (windows-latest)	5m 14s
cargo doc (macos-latest)	1m 57s
cargo clippy + test (macos-latest)	3m 35s
cargo doc (ubuntu-latest)	1m 27s
cargo clippy + test (ubuntu-latest)	3m 53s
	19m 11s

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