differentiator: visitor-simplify: addsub_multiply_by_common_denominator(): process the constant as a fraction with no terms.

Thus it will be easier to implement common denominator multiplication for constants
(i. e. calculate everyone's denominator's LCM and multiply every fraction by it).
differentiator: visitor-simplify: add options; factor out multiplication by common denominator into a separate function.
differentiator: main-taylor: add an alternative main() which solves the original homework assignment in required output format.
differentiator: visitor-simplify: WIP: add product expansion by distributive law of multiplication.

(a + b)(c + d) = ac + ad + bc + bd

This works very, very slow. Don't know why. To be done.