PyAPI: Allow prop collection raw array read access for non-editable props

Non-editable prop collection items would always fall back to the slower
loop in rna_raw_access.

This patch changes RNA_property_collection_raw_array to only fail with
non-editable items when intending to set values, allowing for the faster
raw array access to occur when intending to read values.

This brings the Python API bpy_prop_collection.foreach_get performance
of Mesh.vertex_normals/polygon_normals/corner_normals up to the same
speed as generic FLOAT_VECTOR attributes with the same domains.

Given a mesh with 393216 corners:
Using foreach_get with the "vector" prop and a compatible buffer object:
- Corner vector attribute: ~0.9ms
- Corner normals (before): ~7.9ms
- Corner normals (after):  ~0.9ms
Using foreach_get with the "vector" prop and a Python list:
- Corner vector attribute: ~11.0ms
- Corner normals (before): ~18.0ms
- Corner normals (after):  ~11.0ms

@@ -456,2 +454,2 @@
-                                      PropertyRNA *itemprop,
-                                      RawArray *array);
+int RNA_property_collection_raw_array(
+    PointerRNA *ptr, PropertyRNA *prop, PropertyRNA *itemprop, int set, RawArray *array);