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
Design Task: T76372
This part of a larger refactoring towards a more extensible architecture
in Blender: T75724
The API is defined in `BLO_read_write.h`. It adds the small data structures
`BlendWriter`, `BlendDataReader`, `BlendLibReader` and `BlendExpander`.
Those contain context about the current read/write operation. Furthermore,
it adds many functions with the prefixes `BLO_write_*`, `BLO_read_*` and
`BLO_expand_*`.
Lib linking and expanding will probably be handled by the more generic libquery
system eventually. The corresponding parts of the API can be removed then.
