Add `#[no_std]` and WebAssembly support #62

Job	Run time
rustfmt	6s
clippy	27s
cargo test (stable, false, std,gdk-pixbuf)	1m 18s
cargo test (stable, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 19s
cargo test (stable, false, std,image)	1m 20s
cargo test (stable, false, std,image,fast-linear-to-srgb)	1m 29s
cargo test (stable, false, std,fast-linear-to-srgb)	1m 28s
cargo test (stable, false, std)	1m 36s
cargo test (beta, false, std,gdk-pixbuf)	1m 10s
cargo test (beta, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 12s
cargo test (beta, false, std,image)	1m 20s
cargo test (beta, false, std,image,fast-linear-to-srgb)	1m 20s
cargo test (beta, false, std,fast-linear-to-srgb)	1m 16s
cargo test (beta, false, std)	1m 16s
cargo test (nightly, true)	2m 34s
cargo test (nightly, false, fast-linear-to-srgb)	53s
cargo test (nightly, false)	2m 45s
	22m 49s

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