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
Large objects with many separate pieces became unstably slow
(run for hours and not finish).
The entire original mesh was being duplicated twice per loose part.
In own tests, millions of vertices and thousands of loose parts
now run in around 5-15 seconds.
