The verifier's check in the inner product argument used to assume that the
G'_0 value had an additional (trivial) blinding factor term, which makes
it slightly easier to reason that it never is the point at infinity.
However, we never sample challenges that are zeroes (both for security
and completeness reasons) so this element would never be the point at
infinity anyway. Thus, we can simplify the check with the added benefit of
matching the book's description of the protocol.
This changes variable names in the multiopen and commitment opening implementations
and the book's protocol description to keep names and indicies consistent with one
another.
Co-Authored-By: Jack Grigg <jack@electriccoin.co>