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.
We plan to use the "blender" namespace in other modules as well.
See D6799 for some examples on how to use the library. Reviewers: sergey Differential Revision: https://developer.blender.org/D6799