Since vertex and face normals are now stored on the mesh when necessary,
we can expose them as contiguous arrays of vectors in the Python API.
This can give render engines and other addons easy access to they data
for fast access through a regular collection property.
While "Mesh Vertex" still has a "normal" property in RNA, that is only
maintained in order to avoid breaking the existing API, and accessing
it is less efficient than accessing the normals directly.
I made the normal arrays read-only, because modifying them could
put them in an invalid state. This is inline with how we treat the data
internally, and helps keep relationships between data clear.
Differential Revision: https://developer.blender.org/D13839
b7fe27314b