trivial opts in basic arithmetic with number field elements

Detailed description by Michael Orlitzky:

    The addition to `_reduce_c_()` bails out early if the denominator is
    one because in that case there's nothing to do. The check is cheap
    and avoids several other function calls that do nothing when the
    denominator is one.

    The changes to  `_add_()` and `_sub_()` are identical. We now pull
    out the GCD of the two denominators (which are smallish numbers)
    early, and it eventually cancels out. This would happen anyway when
    we call `_reduce_c_()` before `return`ing, but at that point we
    would be cancelling the same GCD out of the relatively-largish
    product of the denominators. (This also makes it more likely that we
    hit the new "fast return" in `_reduce_c_()`.)

src/sage/rings/number_field/number_field_element.pyx

@@ -2350,6 +2350,8 @@ cdef class NumberFieldElement(FieldElement):
         cdef ZZ_c gcd
         cdef ZZ_c t1
         cdef ZZX_c t2
+        if ZZ_IsOne(self.__denominator):
+            return
         ZZX_content(t1, self.__numerator)
         ZZ_GCD(gcd, t1, self.__denominator)
         if ZZ_sign(gcd) != ZZ_sign(self.__denominator):
@@ -2372,11 +2374,16 @@ cdef class NumberFieldElement(FieldElement):
         """
         cdef NumberFieldElement x
         cdef NumberFieldElement _right = right
+        cdef ZZ_c g, q1, q2
         x = self._new()
-        ZZ_mul(x.__denominator, self.__denominator, _right.__denominator)
+        ZZ_GCD(g, self.__denominator, _right.__denominator)
+        ZZ_div(q1, self.__denominator, g)
+        ZZ_div(q2, _right.__denominator, g)
+        ZZ_mul(x.__denominator, q1, q2)
+        ZZ_mul(x.__denominator, x.__denominator, g)
         cdef ZZX_c t1, t2
-        ZZX_mul_ZZ(t1, self.__numerator, _right.__denominator)
-        ZZX_mul_ZZ(t2, _right.__numerator, self.__denominator)
+        ZZX_mul_ZZ(t1, self.__numerator, q2)
+        ZZX_mul_ZZ(t2, _right.__numerator, q1)
         ZZX_add(x.__numerator, t1, t2)
         x._reduce_c_()
         return x
@@ -2391,11 +2398,16 @@ cdef class NumberFieldElement(FieldElement):
         """
         cdef NumberFieldElement x
         cdef NumberFieldElement _right = right
+        cdef ZZ_c g, q1, q2
         x = self._new()
-        ZZ_mul(x.__denominator, self.__denominator, _right.__denominator)
+        ZZ_GCD(g, self.__denominator, _right.__denominator)
+        ZZ_div(q1, self.__denominator, g)
+        ZZ_div(q2, _right.__denominator, g)
+        ZZ_mul(x.__denominator, q1, q2)
+        ZZ_mul(x.__denominator, x.__denominator, g)
         cdef ZZX_c t1, t2
-        ZZX_mul_ZZ(t1, self.__numerator, _right.__denominator)
-        ZZX_mul_ZZ(t2, _right.__numerator, self.__denominator)
+        ZZX_mul_ZZ(t1, self.__numerator, q2)
+        ZZX_mul_ZZ(t2, _right.__numerator, q1)
         ZZX_sub(x.__numerator, t1, t2)
         x._reduce_c_()
         return x