Replace part of balance verification with multiscalar multiplication operation

[AaronFeickert]

Updates balance proof verification to use multiscalar multiplication operations where applicable.

Review requested.

[levonpetrosyan93]

The idea is good, but I will suggest to leave computation of A as is, as previously it was doing Cout.size() times additions and one multiplication, but now it is doing several multiplications,
I tried to test, and in my setup I got this 716 for old version, 491 for your case, and 400 in case what I am suggesting, numbers are in clock ticks.

[AaronFeickert]

The idea is good, but I will suggest to leave computation of A as is, as previously it was doing Cout.size() times additions and one multiplication, but now it is doing several multiplications,
I tried to test, and in my setup I got this 716 for old version, 491 for your case, and 400 in case what I am suggesting, numbers are in clock ticks.

Updated.

[lgtm-com]

This pull request introduces 2 alerts and fixes 2 when merging 3af6901 into 237d229 - view on LGTM.com

new alerts:

• 2 for Unused static function

fixed alerts:

• 2 for Unused static function