Add `#[no_std]` and WebAssembly support #73

Job	Run time
rustfmt	7s
clippy	6s
cargo test (stable, false, std,gdk-pixbuf)	1m 12s
cargo test (stable, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 4s
cargo test (stable, false, std,image)	1m 8s
cargo test (stable, false, std,image,fast-linear-to-srgb)	1m 4s
cargo test (stable, false, std,fast-linear-to-srgb)	1m 7s
cargo test (stable, false, std)	2m 32s
cargo test (beta, false, std,gdk-pixbuf)	1m 14s
cargo test (beta, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 7s
cargo test (beta, false, std,image)	1m 10s
cargo test (beta, false, std,image,fast-linear-to-srgb)	1m 13s
cargo test (beta, false, std,fast-linear-to-srgb)	1m 8s
cargo test (beta, false, std)	2m 38s
cargo test (nightly, true)	3m 40s
cargo test (nightly, false, fast-linear-to-srgb)	51s
cargo test (nightly, false)	2m 39s
	24m 0s

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