Fixed Rate IRM

The Fixed Rate IRM has a fixed borrowRate maintained by an operator.

function getTrancheBorrowRate(Id id, uint256 trancheIndex, uint256, bytes calldata) external view override(BaseIrm, IIrm)
    returns (uint256 trancheBorrowRate) 
{
    trancheBorrowRate = fixedBorrowRate[id][trancheIndex];
}