Add `#[no_std]` and WebAssembly support #64

Job	Run time
rustfmt	7s
cargo test (stable, false, std,gdk-pixbuf)	1m 28s
clippy	13s
cargo test (stable, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 10s
cargo test (stable, false, std,image)	1m 20s
cargo test (stable, false, std,image,fast-linear-to-srgb)	1m 8s
cargo test (stable, false, std,fast-linear-to-srgb)	1m 16s
cargo test (stable, false, std)	1m 11s
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 11s
cargo test (beta, false, std,image,fast-linear-to-srgb)	2m 5s
cargo test (beta, false, std,fast-linear-to-srgb)	1m 9s
cargo test (beta, false, std)	1m 11s
cargo test (nightly, true)	2m 19s
cargo test (nightly, false, fast-linear-to-srgb)	2m 12s
cargo test (nightly, false)	2m 39s
	23m 1s

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