Change all manual #ifdef to #pragma once

This MR is in a similar vein to !224 (merged) and is part of a series of small updates to the C8 codebase. This MR changes all header file include guards (#ifdef <unique name>) to #pragma once.

In the initial refactoring discussions a few months ago, it seemed there was a consensus to make this change. Every C++17-capable compiler supports #pragma once (GCC added it in 3.4!)