`#pragma once` instead of header guards

[wermos]

Our project currently uses header guards. However, all the compilers we currently support (and wish to support) support the #pragma once directive and have done so for a long time.

The advantages of using #pragma once over traditional header guards are numerous:

• It's more readable
• It's faster to write
• There is no risk of header guard collision (two headers accidentally picking the same guard)

For this reason, I propose moving the project to using #pragma once instead of clunky #ifndef header guards.