Implement plog_complex

Damiano Franzòrequested to merge
damianofranzo/eigen:simd_complex_plog into master

This is the first attempt to deal with vectorized complex functions described in issue #2635

In this commit, I implemented plog_complex. I tested it on my Windows laptop and on my MacBook M1. All the edge cases work std::complex<double> but I get some differences on the last pi (or a fraction of) digits for some corner cases for std::complex<float>.

P.S. This is my first merge request ever in Eigen, any comments are welcome.

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