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
This was the last of the three network optimizations I developed in
the functions branch. Common subnetwork elimination and constant
folding together can get rid of most unnecessary nodes.
Those optimizations work on the multi-function network level.
Not only will they make the network evaluation faster, but they also
simplify the network a lot. That makes it easier to understand the
exported dot graph.
