BLI: add index_range method to some data structures

This can be used to iterate over all indices with less code.

source/blender/blenlib/BLI_array_ref.h

@@ -246,6 +246,11 @@ template<typename T> class ArrayRef {
     return fallback;
   }
 
+  IndexRange index_range() const
+  {
+    return IndexRange(m_size);
+  }
+
   /**
    * Get a new array ref to the same underlying memory buffer. No conversions are done.
    * Asserts when the sizes of the types don't match.
@@ -411,6 +416,11 @@ template<typename T> class MutableArrayRef {
   {
     return ArrayRef<T>(m_start, m_size);
   }
+
+  IndexRange index_range() const
+  {
+    return IndexRange(m_size);
+  }
 };
 
 /**