[SF.12: Prefer the quoted form of #include for files relative to the including file and the angle bracket form everywhere else](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rs-incform)
Additionally changed #include order in `span` so that all `span_ext` is in the GSL include block and not in the STL include block.
Fixes issues #1025.
Co-authored-by: Werner Henze <w.henze@avm.de>
* move span specialization of 'at' to <gsl/span> and update the parameter to be taken by reference
* undid previous changes and acted upon decisions made in maintainer sync. Fixed tests failing in /kernel mode
* ran clang-format on the include folder
* ran clang-format on the test folder
Co-authored-by: Jordan Maples <jordan.maples@microsoft.com>
