[bugfix] fix elusive bug in quo, rem,...

[bugfix] fix elusive bug in quo, rem,...

The power of two rational numeric objects needs not be rational. As a
result, some care is required when transforming (b^e)*l -> (b*l^(1/e))^e
for some rational e and l. This is a common transformation in order to
convert a polynomial over Q into a polynomial over Z when computing
their quotient, remainder, etc. Failure to be careful can potentially
introduce spurious non-rational numbers into rational polynomials and
make those operations fail. This patch avoids this transformation when
l^(1/e) is not a rational number.