Implement `TaffyLayout` widget #61

Job	Run time
cargo fmt	10s
cargo doc (windows-latest)	1m 2s
cargo clippy + test (windows-latest)	2m 5s
cargo doc (macos-latest)	45s
cargo clippy + test (macos-latest)	1m 31s
cargo doc (ubuntu-latest)	34s
cargo clippy + test (ubuntu-latest)	59s
	7m 6s

Provide feedback