make fixed size matrices and arrays trivially_default_constructible

Reference issue

What does this implement/fix?

Make fixed-size matrices and arrays trivially_default_constructible. This requires that Eigen be compiled with EIGEN_NO_DEBUG, i.e. "release mode", or with EIGEN_DISABLE_UNALIGNED_ARRAY_ASSERT. The assert is still useful for scenarios where users are trying to place fixed-size objects in STL containers that don't respect alignment requirements. This also seemed like a good opportunity to get rid of the constructor_without_unaligned_array_assert variants of various constructors sprinkled throughout Eigen.

Big CI: https://gitlab.com/libeigen/eigen_ci_cross_testing/-/pipelines/1473272738

Additional information