Implement `TaffyLayout` widget #16

Job	Run time
cargo doc (windows-latest)	1m 59s
cargo clippy + test (windows-latest)	3m 29s
cargo doc (macos-latest)	1m 14s
cargo clippy + test (macos-latest)	2m 46s
cargo doc (ubuntu-latest)	1m 2s
cargo clippy + test (ubuntu-latest)	2m 8s
cargo fmt	13s
	12m 51s

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