Optimize euclidean distance in host refine phase

[anstellaire]

Issue

Original code (below) generated serial assembly and used strictly-ordered fadda instruction on ARM with gcc & clang. That resulted in suboptimal performance.

for (size_t k = 0; k < dim; k++) {
  distance += DC::template eval<DistanceT>(query[k], row[k]);
}

Proposed solution

This PR provides euclidean distance optimized with partial vector sum (below), that helps vectorization but loses strcictly-ordered compliance.

template <typename DC, typename DistanceT, typename DataT>
DistanceT euclidean_distance_squared_generic(DataT const* a, DataT const* b, size_t n) {
  size_t constexpr max_vreg_len = 512 / (8 * sizeof(DistanceT));

  // max_vreg_len is a power of two
  size_t n_rounded = n & (0xFFFFFFFF ^ (max_vreg_len - 1));
  DistanceT distance[max_vreg_len] = {0};

  for (size_t i = 0; i < n_rounded; i += max_vreg_len) {
    for (size_t j = 0; j < max_vreg_len; ++j) {
      distance[j] += DC::template eval<DistanceT>(a[i + j], b[i + j]);
    }
  }

  for (size_t i = n_rounded; i < n; ++i) {
    distance[i] += DC::template eval<DistanceT>(a[i], b[i]);
  }

  for (size_t i = 1; i < max_vreg_len; ++i) {
    distance[0] += distance[i];
  }

  return distance[0];
}

In addition, it has an implementation with NEON intrinsics which provides further speedup on certain test cases (can be removed if arch-specific code is undesired).

Results

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[cjnolet]

/ok to test

[anstellaire]

/ok to test

UPD:
@cjnolet, seems like CI is triggered only by repository members, could you please do it one more time?
I changed formatting with clang-format.

[lowener]

/ok to test