Add no_std support to remaining crates

[bushrat011899]

Objective

no_std support improves compatibility with the Rust ecosystem, in everything from Rust-for-Linux through to embedded and high-compatibility WASM.

Solution

• Added no_std to image-canvas
• Added no_std to image-drm
• Added libm dependency to image-canvas for float operations without std. This is a high-quality library provided by rust-lang themselves.
• Added a runtime-features feature to image-canvas for performing runtime feature detection (requires std).
• Added compile-time feature detection, allowing architecture specific optimisations to be used without std.

---

Notes

• This PR is mostly inspired by this comment.
• I've added a little more structure to how ShuffleOps brings in architecture specific optimisations. This helps with feature flags and should be easier to maintain.
• I chose not to add a libm feature as I suspect it may be required even more-so in the future of this crate. The alternative of inlining a powf implementation or additional conditional compilation seems undesirable to me.

[197g]

I've added a little more structure to how ShuffleOps brings in architecture specific optimisations. This helps with feature flags and should be easier to maintain.

❤️

[197g]

Actually I don't expect this, this is an artifact of ongoing development work and should get linted as a nit that it needs to go, or be used (before the next release). Same with the Yuv420p` layout and so on. Rather have those as a separate PR (please amend the commit instead of just adding one on top, to avoid merge conflicts with my branch that does touch some of these lints).

[bushrat011899]

Tried editing that particular commit directly but I think that didn't work as you'd like. If there's any issues with the history as-is just let me know and I can re-open this PR on a clean branch instead.

[197g]

I can also squash-and-merge so no worries, it'd just have been neater.

[197g]

So if you're not comfortable with that workflow in git before you can also just add commits and let me know to squash. We lose the history of individual commits but should be the same result.

[197g]

Thank you a lot for the feedback on no_std needs. Went ahead and did the minor nits. All the activity around me at RustWeek is making me uncharacteristically wired.