ERC721 Support in BatchReader

Add the function batchERC721Balances in BatchReader to grab all NFTs owned by tokenHolders in tokens.

Description

The batchERC721Balances accepts two arguments, address[] tokens and address[] tokenHolders. The function will return a matrix of NFTs owned by each tokenHolder in each token.

Motivation and Context

To avoid loop calls to RPC nodes to gather this data

How Has This Been Tested?

This was tested using a unit test that deploys 2 ERC721 tokens, mints several tokens to 3 different addresses and then uses a single batchERC721Balances call to get all data for both tokens

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[x] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly.
[x] I have added tests to cover my changes.
[x] All new and existing tests passed.

Consensys / UniversalToken