Adding basic support for "curve" functions
Some example "curve" functions

Of note is using a table in program memory to store a lookup table and the implemented exponential curve

Usage:

LEDFader::curve_function current = someLEDFader.get_curve();

if (!current || current != &Curve::exponential) // Yay redundancy!
 someLEDFader.set_curve(&Curve::exponential); // This curve rules.