Reverts commit that changes #include "assert" -> #include "gsl/assert".
This change is necessary in order to comply with CppCoreGuideline's
SF.12. Now we do #include "./assert".
Office is seeing build breaks due to `#include "span"` including
C++20 span instead of gsl/span. Most likely we want all headers
includes qualified with "gsl/" to avoid similar issues.