Add `#[no_std]` and WebAssembly support #63

Job	Run time
clippy	8s
rustfmt	9s
cargo test (stable, false, std,gdk-pixbuf)	1m 23s
cargo test (stable, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 12s
cargo test (stable, false, std,image)	1m 14s
cargo test (stable, false, std,image,fast-linear-to-srgb)	1m 10s
cargo test (stable, false, std,fast-linear-to-srgb)	1m 7s
cargo test (stable, false, std)	1m 20s
cargo test (beta, false, std,gdk-pixbuf)	1m 19s
cargo test (beta, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 10s
cargo test (beta, false, std,image)	1m 15s
cargo test (beta, false, std,image,fast-linear-to-srgb)	1m 14s
cargo test (beta, false, std,fast-linear-to-srgb)	1m 41s
cargo test (beta, false, std)	1m 27s
cargo test (nightly, true)	1m 41s
cargo test (nightly, false, fast-linear-to-srgb)	1m 41s
cargo test (nightly, false)	1m 39s
	20m 50s

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