Add StableDeref impls for no_std types, including alloc and spin types

[kevinaboos]

As a no_std Rustacean, I needed this for my own purposes. But I figured it'd be pretty useful for other people too, especially if they're using owning_ref in their projects.

Happy to make any changes you'd prefer. One thing is that I feel the way you set the default-features is a bit confusing and can lead to weird combinations of features, but I left it as is because I didn't want to break other crates using it.

The new features are used like this:
cargo build --no-default-features --features alloc
cargo build --no-default-features --features spin

[Storyyeller]

So just to clarify, alloc is a built in nightly-only crate, while spin is a third party crate, right? I think in the case of spin, it would make more sense to ask the author of spin to add the impls in that crate.

[kevinaboos]

That's correct. Regarding spin, many other crates have dependencies on that and treat it as always available (just like std's Mutex types), and even have features that allow you to use spin in place of standard Mutexes, such as lazy_static. So it wouldn't be a stretch to include it in this crate because its often treated as a built-in type.

You'd either be including an optional dependency on spin in this crate, or including an optional dependency on this crate in spin. I'm not really sure what's more idiomatic...

[Storyyeller]

Seeing as this crate is literally just a trait definition, I think it makes sense to avoid dependencies. There's no reason why spin can't include an optional dependency on it.

[kevinaboos]

Works for me, I'll remove the spin-related parts.

[philipc]

@kevinaboos I have need of the alloc support. Will you be able to update this PR?

[kevinaboos]

@philipc ah, I completely forgot about this, got swamped at work. I'll update the PR hopefully within a day or two at the most.

[philipc]

Does this need to be #![cfg_attr(feature = "alloc", feature(alloc))]?

[kevinaboos]

it's not technically required, but it's more correct that way. Changed, thanks!

[kevinaboos]

Okay, I think it's ready for merging now, @Storyyeller.

[Storyyeller]

Ok, I'll try to get to it over the weekend.