This merges changes in internals, runtime-only of existing custom
normals code, which make sense as of themselves, and will make diff of
soc branch easier/lighter to review.
In the details, it mostly changes two things:
* Now, smooth fans (aka MLoopNorSpaceArray) can store either loop
indices, or pointers to BMLoop themselves. This makes sense since in
BMesh, it's relatively easy to get index from a BMElement, but nearly
impracticable to go the other way around.
* First change enforces another, now we cannot rely anymore on `loops`
being NULL in MLoopNorSpace to detect single-loop fans, so we instead
store that info in a new flag.
Again, these are expected to be totally non-functional changes.
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