Functions: improve devirtualization in multi-function builder

This refactors how devirtualization is done in general and how
multi-functions use it.

* The old `Devirtualizer` class has been removed in favor of a simpler
  solution. It is also more general in the sense that it is not coupled
  with `IndexMask` and `VArray`. Instead there is a function that has
  inputs which control how different types are devirtualized. The
  new implementation is currently less general with regard to the number
  of parameters it supports. This can be changed in the future, but
  does not seem necessary now and would make the code less obvious.
* Devirtualizers for different types are now defined in their respective
  headers.
* The multi-function builder works with the `GVArray` stored in `MFParams`
  directly now, instead of first converting it to a `VArray<T>`. This reduces
  some constant overhead, which makes the multi-function slightly
  faster. This is only noticable when very few elements are processed though.

No functional changes or performance regressions are expected.

source/blender/geometry/intern/mesh_to_curve_convert.cc

@@ -2,7 +2,6 @@
 
 #include "BLI_array.hh"
 #include "BLI_array_utils.hh"
-#include "BLI_devirtualize_parameters.hh"
 #include "BLI_set.hh"
 #include "BLI_task.hh"