This commit replaces our Vc backend with xsimd, QuantStack's SIMD
offering backing the xtensor project.

This requires the following:

- wholesale replacement of Vc with xsimd alternatives
- implementation of dispatching against the xsimd::generic architecture
  (i.e. Vc::ScalarImpl)
- implementation of rudimentary gather-scatter and integer rounding ops

This brings Arm support to our backend for Free(tm).

Co-Authored-By: Dmitry Kazakov <dimula73@gmail.com>

CCMAIL: kimageshop@kde.org