Update commit to accept references

[hansieodendaal]

It's unfortunate that the BP+ API forces copies :(

There are a few ways to remove this need:

Update commit in the BP+ API to accept I: Iterator<Item=&Scalar>, then you can pass in k_i.iter().map(|k| &k.0)

Or take something like

pub fn commit<S: AsRef<Scalar>>(&self, value: Scalar, blindings: &[S]) -> Result<P, ProofError> 

Either way, it requires changes to the BP+ repo, which is not something we can do in this PR.

Originally posted by @CjS77 in tari-project/tari-crypto#99 (comment)

[hansieodendaal]

See #12