This also renames `MutableArrayRef` to `MutableSpan`.
The name "Span" works better, because `std::span` will provide
similar functionality in C++20. Furthermore, a shorter, more
concise name for a common data structure is nice.
Allows to access dependency graphs created for render engines
to inform them about changes in .blend file structure from the
Python handlers.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D5724
