calc() expression reducer

[c9s]

While SASS is interpreing this as it is:

$some: 22;
width: calc(100% - 10px + 20px + $some);

We can reduce the expression if they are the same unit to:

width: calc(100% + 10px + 22);

[c9s]

Add FunctionCall.IsLiteral() method to check if we need to present the function call as literal string.