fix: return variable names consistency

[wei3erHase]

Addressing @turtlemoji comments on return variable name consistency, this is the approach that was chosen:

• maintain methods as they were, with return variable names w/o explicit returns:

function myMethod() public returns (uint256 myReturn) {
    myReturn = calc();
    // notice: no `return myReturn`
}

• removing all obvious view method return variable names (for explicit returns):

function getValue() public returns (uint256) {
    // notice: no `_myValue = value`
    return value;
}

• BMath contract was deliberately chosen to maintain the named returns (despite being views)
• non-obvious view returns were also chosen to be named, such as:

function getValidSignature() public returns (uint256 magicValue){
    magicValue = magic();
    // notice: this one could have been `return magic();`
}