Dont use std, when avoidable

[hellow554]

I try to use this crate for an embedded platform, where I dont have std, but only core. Would it be possible to get rid of std and rely on core instead?

[hellow554]

I tried to implement it in my branch (see https://github.com/punkkeks/enum_traits/commits/master ), the problem is, that I had to remove the EnumIsVariantFns, because it uses std::ascii, which is not in core.
Because I don't need it in my current project, I just removed it and was able to compile and use it.

Maybe it could be a starting point to further development

[Lolirofle]

I made it a Cargo feature by looking at your branch, but I am not sure it works properly. Is it possible for you to test it and see if it works for you? The "no_std" feature is documented in the README.