Performance tracking issue

[Geordon Worley]

Performance is currently only beating linear (which is incredibly fast on modern processors) for exact search at > 1M features. This is probably beatable, but it will require some optimizations. Below is the list of optimizations and their issues that need to be made:

k-NN

• Use packed_simd to speed up FeatureHeap::add
• Convert search module to use a hand-optimized iterator for each level of the tree
• Investigate if the search module can be sped up using SIMD or bit twiddling
• Implement more SIMD optimizations upstream in swar

[Geordon Worley]

After closing #7 I am now getting the following:

neighbors/nearest_1_hwt_maxerr_0/16777216                        
                        time:   [17.479 ms 17.687 ms 17.849 ms]

This is a reasonable speedup over linear at 2^24 features.