Fix modular reduction during PolyRatFun expansion

[cbmarini]

PolyRatFun expansion does not consistently reduce coefficients after its arguments have been reduced modulo the active modulus. For example:

Modulus 13;
CFunction rat;
Symbol ep;
PolyRatFun rat(expand,ep,2);

Local F = rat(1,1+ep/2);
Print +s;
.end

This produces rat(1 - 7*ep + 49*ep^2) instead of rat(1 + 6*ep + 10*ep^2).

This PR adds the modular reduction to ExpandRat(), InvPoly() and MultiplyWithTerm(), which perform the PolyRatFun expansion. Furthermore, also related PolyFun and dollar factorization paths are fixed. Regression tests and the manual are updated.

[coveralls]

Coverage is 66.128% — cbmarini:polyfun-modulus into form-dev:master. No base build found for form-dev:master.