Potential improvements for System.Numerics.Complex

[lilinus]

I have found a number of potential places for improvements in System.Numerics.Complex:

1. Use Math.SinCos when available in various places.
2. Use double.Hypot when available in Complex.Hypot
3. sinh and cosh are calculated simultaneously as:

double p = Math.Exp(x);
double q = 1 / p;
double sinh = (p - q) * 0.5;
double cosh = (p + q) * 0.5;

This is inaccurate for sinh due to catastrophic cancellation of p-q when x is close to zero.
Could perhaps be solved by using Math.Sinh and Math.Cosh within a threshold?

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[tannergooding]

Please feel free to submit a PR for each of these.

For 3, we should just always call Math.Sinh and Math.Cosh to avoid potential inaccuracies and ensure IEEE 754 compliant results.