Add `#[no_std]` and WebAssembly support #72

Job	Run time
rustfmt	3s
clippy	21s
cargo test (stable, false, std,gdk-pixbuf)	1m 23s
cargo test (stable, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 24s
cargo test (stable, false, std,image)	1m 22s
cargo test (stable, false, std,image,fast-linear-to-srgb)	1m 31s
cargo test (stable, false, std,fast-linear-to-srgb)	1m 27s
cargo test (stable, false, std,fast-linear-to-srgb,wasm)	25s
cargo test (stable, false, std,wasm)	35s
cargo test (stable, false, std)	2m 46s
cargo test (beta, false, std,gdk-pixbuf)	1m 15s
cargo test (beta, false, std,gdk-pixbuf,fast-linear-to-srgb)	1m 21s
cargo test (beta, false, std,image)	1m 16s
cargo test (beta, false, std,image,fast-linear-to-srgb)	1m 16s
cargo test (beta, false, std,fast-linear-to-srgb)	1m 18s
cargo test (beta, false, std,fast-linear-to-srgb,wasm)	27s
cargo test (beta, false, std,wasm)	24s
cargo test (beta, false, std)	2m 40s
cargo test (nightly, true)	3m 10s
cargo test (nightly, false, fast-linear-to-srgb)	49s
cargo test (nightly, false, wasm)	53s
cargo test (nightly, false)	2m 31s
	28m 37s

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