fix: avoids copying in the commit method

[therustmonk]

The PR improves PedersenGens::commit method:

• Internal copying: avoid copying values inside the method (use iters)
• External copying: avoid copying on calls (support AsRef<Scalar> args)

UPD: The Borrow trait used (instead of AsRef), because it's the requirement of the multiscalar_mul method in the curve25519-dalek crate.

[stringhandler]

Nice

[hansieodendaal]

LGTM - solves #9
(tests need to be updated, .commit( in tests needs a reference, as per cargo clippy --all-targets -- -D warnings)