Don't use serde default features (std) for spirv crate

[Dinnerbone]

Despite spirv crate being no_std, it pulls in serde's std feature by accident. This fixes that, so it can be used safely in no_std crates

[MarijnS95]

I'm curious how this got to be (but git blame isn't readily available on mobile). Most likely the crate used to be no_std and serde integration was added at some later time?

In any case I don't think we should land this in isolation, but rather spend some time adding a no_std check to CI as well as the relevant clippy lints that guide contributors to use core primitives over std.

[MarijnS95]

Looks like serde was added in #125, but the dependency is optional behind a non-default feature meaning that spirv is already no_std compliant unless the serialize or deserialize features are turned on.

To that effect, I think you should rewrite your pull request title and description to explain that you have tested and confirmed that iff the (de)serialize features are used, this crate could still be completely no_std-compatible while also providing serde support.