no_std support

[olanod]

This PR turns the crate into no_std compatible replacing std references with core + alloc and adding the std feature where needed(std::error::Error support).
Also tried to make the dependency on HashMap optional making extensions an optional feature, the ahash dependency shouldn't be an issue since it's already a dependency of hashbrown which is the the std HashMap implementation.

[jplatte]

TypeId implements Ord, so how about using BTreeMap for extensions if there is no std, rather than not having it?

[algesten]

This looks really good. Wonder if the crate authors will get time to consider it?

[algesten]

Hm. I guess it doesn't get rid of allocations cause it relies on the extern crate alloc

[seanmonstar]

I'm not sure about getting this into 1.0, but I do want to allow for it. I think the only breaking change would be to add a std feature that is default-on, yea? Then we could eventually fix up support with it off.

[algesten]

Yes. I got a feeling i've seen more std feature flags than the opposite no_std. Either way works though.

[seanmonstar]

I created #637 which does that, allowing us to eventually add support later.