`no_std` support

[AverageHelper]

most of the things this crate does relies on std's re-exports from core and alloc, so #![no_std] can be added already with only a few tweaks!

the new std feature only turns on serde's std feature, it doesn't do anything to this crate.

I ran the tests the same as they are in the travis config, and tested locally against another no_std crate of mine, so hopefully that means things Just Work™ after this :sweat_smile: