Add `#[no_std]` and WebAssembly support #69

Job	Run time
rustfmt	6s
clippy	11s
cargo test (stable, false, std,gdk-pixbuf)	1m 19s
cargo test (stable, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 16s
cargo test (stable, false, std,image)	1m 32s
cargo test (stable, false, std,image,fast-linear-to-srgb)	1m 19s
cargo test (stable, false, std,fast-linear-to-srgb)	1m 6s
cargo test (stable, false, std)	1m 48s
cargo test (beta, false, std,gdk-pixbuf)	1m 21s
cargo test (beta, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 11s
cargo test (beta, false, std,image)	1m 10s
cargo test (beta, false, std,image,fast-linear-to-srgb)	1m 16s
cargo test (beta, false, std,fast-linear-to-srgb)	1m 16s
cargo test (beta, false, std)	1m 50s
cargo test (nightly, true)	1m 49s
cargo test (nightly, false, fast-linear-to-srgb)	1m 48s
cargo test (nightly, false)	1m 49s
	22m 7s

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