Use the BMesh symmetrize operator instead of using the modifier code.
While we could support shape-keys with the existing code used by the
mirror modifier, we'd need to add code-paths for evaluated mesh & bmesh
conversion to handle shape-keys differently just for this one case,
since we want to avoid copying & processing shape-keys layers for
evaluated meshes in general.
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