Derive rather than impl `Default` for `Fill` #545

Job	Run time
formatting	27s
cargo clippy (windows-latest)	50s
cargo clippy (macos-latest)	35s
cargo clippy (ubuntu-latest)	32s
cargo test (macos-latest)	36s
cargo test (wasm32)	19s
cargo test (windows-latest)	46s
cargo test (ubuntu-latest)	17s
cargo check (msrv) (windows-latest)	59s
cargo check (msrv) (macos-latest)	27s
cargo check (msrv) (ubuntu-latest)	20s
cargo doc	20s
cargo check (msrv) (wasm32)	22s
cargo clippy (wasm32)	28s
typos	6s
	7m 24s

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