Returns numerator of rational number in canonical form.
Given a rational number as supported by the Num library, numerator returns
the numerator $p$ from the rational number cancelled to its reduced form, $p/q$
where $q > 0$ and $p$ and $q$ have no common factor.
# numerator(Int 22 // Int 7);;
val it : num = 22
# numerator(Int 0);;
val it : num = 0
# numerator(Int 100);;
val it : num = 100
# numerator(Int 4 // Int(-2));;
val it : num = -2