Fix "unary minus operator applied to unsigned type, result still unsigned" on MSVC and other stupid warnings

Reference issue

What does this implement/fix?

I fixed a few irritating compiler warnings that only seem to be important to MSVC, including:

• negating unsigned integers. On gcc/llvm, the negation operator -x seems to be synonymous with 0 - x (for integers). I make that explicit in Eigen.
• Various implicit cast from real to std::complex warnings which are suppressed by casting to RealScalar instead of Scalar
• avx512 pblend warnings
• warnings in unit tests

Additional information