Tricks the compiler into skipping expensive
`uninit var analysis` (`-Wmaybe-uninitialized`) by using
a struct with state rather than separate variables for `best` /
`bestDiff`.
This has no performance impact.
Also optimizes lookup a bit further and moves some code that
does not need to be inlined to the cpp file.
```
Benchmark Time CPU Time Old Time New CPU Old CPU New
-----------------------------------------------------------------------------------------------------------------------------------
BM_GenerateBlendedLookupTable_pvalue 0.0002 0.0002 U Test, Repetitions: 10 vs 10
BM_GenerateBlendedLookupTable_mean +0.0237 +0.0237 2092090 2141601 2091732 2141291
BM_GenerateBlendedLookupTable_median +0.0237 +0.0237 2092104 2141662 2091669 2141319
BM_GenerateBlendedLookupTable_stddev -0.6414 -0.5834 664 238 538 224
BM_GenerateBlendedLookupTable_cv -0.6497 -0.5930 0 0 0 0
BM_BuildTree_pvalue 0.0002 0.0002 U Test, Repetitions: 10 vs 10
BM_BuildTree_mean +0.0410 +0.0410 4495 4679 4494 4678
BM_BuildTree_median +0.0403 +0.0402 4494 4675 4493 4674
BM_BuildTree_stddev +0.9515 +0.9359 7 14 7 14
BM_BuildTree_cv +0.8746 +0.8596 0 0 0 0
BM_FindNearestNeighbor_pvalue 0.0002 0.0002 U Test, Repetitions: 10 vs 10
BM_FindNearestNeighbor_mean -0.0399 -0.0398 1964257108 1885966812 1963954917 1885694336
BM_FindNearestNeighbor_median -0.0397 -0.0396 1963969748 1886074435 1963650984 1885803182
BM_FindNearestNeighbor_stddev -0.3380 -0.3443 1217360 805946 1225442 803469
BM_FindNearestNeighbor_cv -0.3105 -0.3171 0 0 0 0
OVERALL_GEOMEAN +0.0077 +0.0077 0 0 0 0
```
Gleb Mazovetskiy