Add helper methods NaN/Infinity for Complex type

[kant2002]

Overview

When working with Complex numbers there now helper methods for dealing with NaN and Infinity values. For example with double it is possible check is value is NaN or Infinity

API

This is an example of how the API could look like.

public class Complex
{
    public static bool IsFinite(Complex value);
    public static bool IsNaN(Complex value);
    public static bool IsInfinity(Complex value);

    public static Complex  NaN { get; }
    public static Complex  Inifinity { get; }
}

/cc: @tannergooding

[tannergooding]

What is the proposed value for Infinity and NaN?

[kant2002]

I'm almost sure that only one sane way to return NaN is to return (NaN, NaN). For Infinity is more likely personal opinion. I think that current implementation of Complex really model just single Infinity which happens to be represented in different ways. so I propose (+Inf, +Inf)

[tannergooding]

Resolved in dotnet/corefx#33184