Instantiate SIMD Ops, not distance implementations

For ISA dispatching, we instantiate the distance implementations (`L2Impl`, `IPImpl`, `CosineSimilarityImpl`). Since there are template parameters `N, Ea, Eb, AVX_AVAILABILITY`, corresponding to dimensionality, element type A, element type B, and AVX availability flag, respectively, many combinations of explicit template values are required. Crucially, also one for `N`, the fixed-/dynamic-dimensionality support.

It is not the best place to create the AVX-specific instantiations, because only the actual compute ops benefit from ISA-optimization.

The current implementations creates a lot of code
* at the end of the distance headers (`cosine.h`, `euclidean.h`, `inner_product.h`) to define the symbols as `extern`; and
* to produce the instantiations `multi-arch/avx2.cpp` and `multi-arch/avx512.cpp`;
all of which makes heavy use of preprocessor macros and is therefore hard to understand and debug.

Completion of #183 should allow to lower the explicit instantiations to the various compute ops (`IPFloatOp`, `IPVNNIOp`, `L2FloatOp`, etc.), for which we already use fixed values of `N`, eliminating combinatorial complexity in the instantiations.

The task is to understand if is after completion of #183 such a simplification is possible, and, if so, implement it.

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AS A maintainer of SVS
I WANT TO lower the instantiation of architecture-specific functions to the actual compute operations
SO THAT I can reduce maintenance and future development cost, as well as have a more optimal solution in general.