Fix incorrect calculation produced by acotan(num)

The num version of the method uses incorrect formula to calculate acotan.
The following identities should hold:
    θ = acotan cotan θ for -π < θ < π
    θ = cotan acotan θ for θ ∈ ℝ

Fix by calculating atan(1/x), result of which is corroborated by the above
identities, WolframAlhpa[^1], and our implementation of acotan for boxed types.

[1] http://www.wolframalpha.com/input/?i=acotan

src/core/Num.pm

@@ -493,7 +493,7 @@ multi sub cotan(num $x) returns num {
     nqp::div_n(1e0, nqp::tan_n($x));
 }
 multi sub acotan(num $x) returns num {
-    nqp::div_n(1e0, nqp::atan_n($x));
+    nqp::atan_n(nqp::div_n(1e0, $x));
 }
 multi sub sinh(num $x) returns num {
     nqp::sinh_n($x);