T::zero(), T::one(), etc should be const fn

[jgarvin]

It's pretty hard to use the num crate for writing generic const fns without this. Admittedly only useful currently with #![feature(const_fn_trait_bound)] but I don't think there is any harm to making them const fn now? Would require version bump though.

[cuviper]

What would that do to types that can't be const, like BigInt?

[jgarvin]

Ah, I've only been using for abstraction over primitive types. Maybe PrimInt can have supertraits ConstOne and ConstZero? I don't know if Rust provides any way to avoid duplicating traits for every operation.