Blender Git Loki

BLI: add reverse iterators, iterator constructor and Vector.insert/prepend

The new reverse iterators behave as the reverse iterators for contains from
the standard library. Have a look at the tests to see how to use them.
Using them will hopefully become easier with ranges in C++20.

A Vector can now be constructed from two iterators, which is very common
in the standard library.

New Vector.insert methods allow adding elements in the middle of a vector.
These methods should not be used often in practice, because they has a linear running time.

New Vector.prepend methods allow adding elements to the beginning of a vector.
These methods are O(n) as well.