Bug 13022 – std.complex lacks a function returning the squared modulus of a Complex

std.complex has the function `pure nothrow @safe T abs(T)(Complex!T z);`, which returns the modulus of a Complex!T, but lacks a function that returns its squared modulus. Using abs(z)*abs(z) would be inefficient due to the computation of the square root. I suggest adding a function `pure nothrow @safe T abs2(T)(Complex!T z);`, as well as `pure nothrow @safe T abs2(T)(T z);` for genericity. The latter would simply be x -> x*x. This function would be equivalent to C++ std::complex::norm. The name `abs2` is used for consistency with `abs`, and is less mathematically incorrect than `norm`.

Pull request: https://github.com/D-Programming-Language/phobos/pull/2297