Add method for randomly sampling non-zero elements

[ValarDragon]

We should add a method either on UniformRand or on a trait thats auto-derivable for traits with UniformRand and Zero, in order to sample non-zero elements

Originally posted by @ValarDragon in arkworks-rs/groth16#16 (comment)

[burdges]

You might glance at the distributions stuff from the rand crate, probably not quite right here but worth a quick look.