Blender Git Loki

Cleanup: Move weld modifier to C++

This moves `MOD_weld.cc` to C++, fixing compiler warnings
coming from the change. It also goes a little bit further and converts
the code to use C++ data structures: `Span`, `Array`, and `Vector`.
This makes the code more shorter and easier to reason about, and
makes memory maneagement more automatic.

Differential Revision: https://developer.blender.org/D13618