Implement support for no_std

[wackbyte]

I added a std feature to support building with no_std.

If enabled, it implements the Error trait and uses std's HashSet.
Otherwise, it doesn't implement Error and uses hashbrown's HashSet.

However, there are a few details I'd like to hear your opinion on if I need to make some changes.

• no_std is disabled in documentation.
• Because of Optional dependency when a feature is *not* enabled. rust-lang/cargo#1839, the dependency on hashbrown can't be optional. It could probably be if there was a no_std feature instead, but according to the feature reference in the Cargo book, features should be additive, and a std feature should be used over a no_std one.
• I looked into hashbrown and found that it uses ahash by default (through a default feature), so I removed the direct dependency on it and used it through hashbrown. This may be undesirable.
• The Verbose struct in the debug module currently uses the print! macros. Since those do not exist in a no_std environment, the function that uses them was changed to be a no-op when std is disabled.

(Closes #38)

[zesterer]

I think, as you mention in the post, that this might be the wrong way to go about it. Cargo features are strictly additive, and so there should be an std feature as part of the default-features list instead. This should also solve the issue you encountered with hashbrown, I believe.

What do you think about this?

[wackbyte]

I did use a std feature and added it to default-features, but the issue is that I can't enable an optional dependency if a feature is disabled.

[zesterer]

This might require hashbrown being non-optional in this case, which is a shame. I'd prefer that over a 'negative' feature flag though, I think.

[zesterer]

Hey, this has a few conflicts. Could you resolve them? Happy to merge after that.

[zesterer]

Thanks very much! I'll probably try to find time to add a no_std target to the CI tomorrow (some nebulous embedded ARM platform will do) so we can make sure that nothing that relies on std accidentally sneaks in without the appropriate flag.

[mainrs]

Glad you could work it out @wackbyte. I didn't get the hasher to compile properly. Your ahash feature and implementation was missing in my PoC.

I should have updated the issue with my progress and problem though, maybe it could have been solved that way!
Thanks for the PR. chumksy on wasm is something I have been looking forward to!

[zesterer]

Great to hear that the library fits this use-case. I'm interested to hear how it works out!