Introduce precompiled headers (on Windows)

Precompiled headers (PCHs) exist on both Windows and Linux. For Linux, you can refer to https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html. Straight from the GNU CC documentation: "The time the compiler takes to process these header files over and over again can account for nearly all of the time required to build the project."

PCHs are only being proposed for the CMake system. In theory, we can introduce this change with only a few, non-intrusive code changes. The feature will primarily be a CMake change.

See: https://github.com/sakra/cotire