[Issue 13022] std.complex lacks a function returning the squared modulus of a Complex

[Element 126 via Digitalmars-d-bugs]

Before submitting my pull request, I would like to hear your opinion 
about the naming of the squared modulus function, since we won't be able 
to change it once it is released.
I picked `abs2` for consistency with `abs` and because it is 
mathematically more accurate than C++'s `norm`.
What do you think of it ?