multicallAllowance method
Multicall of allowance, may not be in the same block.
Implementation
Future<List<BigInt>> multicallAllowance(
List<String> owners, List<String> spenders) async {
assert(owners.isNotEmpty, 'Owner list empty');
assert(spenders.isNotEmpty, 'Spender list empty');
assert(owners.length == spenders.length,
'Owner list length must be same as spender');
return Future.wait(Iterable<int>.generate(owners.length).map(
(e) => allowance(owners[e], spenders[e]),
));
}