Avoid unnecessary indirect includes for the module, which would slow
down compilation. Avoid "using std" which is generally not considered
good practice since it's helpful to see what namespace things are from.
This is just a general cleanup of the area. I was trying to understand
it better to reduce redundant mesh topology storage but finding that
some intermediate cleanups would be helpful to ease the change.
Pull Request: https://projects.blender.org/blender/blender/pulls/120743
6a21ff9154