differentiator: visitor-simplify: insert addsub's constant to the front.
differentiator: visitor-simplify: don't exit folding when there is no nodes; we still have to do "sign folding".
differentiator: main: do not print "Partial derivatives:" header when there is no partial derivatives to show.
differentiator: main: implement building Taylor series for arbitrary expressions.

For now only in neighborhood of 0 (Maclaurin series).
differentiator: main, node, visitor: prepare for arbitrary precision arithmetics -- work around bug/ugliness in implicit conversion.
util: variable: prepare for arbitrary precision arithmetic -- work around bug in boost or iostream.
c: prepare for arbitrary precision arithmetic and fix unnoticed bug after conversion to rational arithmetic.
util: enable arbitrary precision arithmetic.