Add no_std support

[AverageHelper]

I'm hoping to use this crate in a no_std package, but the references to std cause compilation failure. Since some structures, such as NESlice and NEVec, don't rely on std directly, only things re-expored from alloc or core, only a minor refactor of imports and some feature guards are necessary for this crate to be usable in embedded environments.

I tested these changes locally in another crate, and compiled against a target which has no std support (thumbv6m-none-eabi), unit tests seem to pass, so hopefully it all works! (Apologies in advance for the slight added maintenance burden of needing to import against core and alloc instead of std 😅)

I also added CI tests to check that this continually builds in no_std and non-alloc environments. (Both against the same target, since I'm not yet sure of a built-in target where core exists but not alloc.)