Increase reusal of specialized SIMD classes

[edanor]

Some SIMD types, such as SIMD1 and SIMD2 will have similar if not the same implementation for different plugins. It would be very useful to have some code that can be reused.

One of the problems with code reuse is separate naming for classes of different plugins.

[edanor]

Refactored AVX and AVX2 plugins.

[edanor]

Refactored AVX512 and KNC plugins.

Scalar emulation plugin should also be present as a separate plugin with specialized implementations similar to SIMD1 and SIMD2. This should further simplify code for auto-vectorization, reduce compilation time (smaller number of indirections) and create a common base for specialized emulated classes in other plugins.

[edanor]

Refactored scalar emulation plugin.

[edanor]

All plugins now have separate file trees. This should increase re-usability of specific specialization classes, code readability and would make the code easier to maintain.