This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.
This patch has been generated by P1561 followed by `make format`.
Differential Revision: https://developer.blender.org/D8466
Allows to use pre-defined structure for session UUIDs in all data
structures which needs it: pose channels, sequencer strips, modifiers.
The goal of all this is to have a reliable way of matching original
and copy-on-written versions of data, so that it's possible to
preserve runtime caches.
