Implement `(Premul)Rgba8::from_u32` and `from_u8_array` #483

Job	Run time
cargo clippy (wasm32)	23s
formatting	31s
cargo clippy (windows-latest)	1m 8s
cargo test (windows-latest)	41s
cargo clippy (macos-latest)	25s
cargo test (macos-latest)	20s
cargo clippy (ubuntu-latest)	29s
cargo test (ubuntu-latest)	14s
cargo check (msrv) (wasm32)	22s
cargo check (msrv) (windows-latest)	40s
cargo doc	11s
cargo check (msrv) (macos-latest)	23s
cargo check (msrv) (ubuntu-latest)	17s
cargo miri	1m 43s
cargo test (wasm32)	14s
typos	6s
	8m 7s

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