Use google benchmarks and fix minor performance issues

[alexshalamov]

PR has following changes:

• Rewrite benchmarks to use google benchmarks
• Add few simple fixes (emplace, move, reserve)
• Avoid expensive unordered_map copies and use shared_ptr internally while creating an index

Benchmarks (2.3 GHz 8-Core Intel Core i9, macbook pro)

Comparing ./build/bench_head to ./build/bench
Benchmark                                                           Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------------
ParseGeoJSON                                                     +0.0017         +0.0018          2272          2275          2271          2275
GenerateTileIndex                                                -0.2651         -0.2649         11567          8501         11558          8496
TraverseTilePyramid/iterations:3                                 -0.1701         -0.1700          1332          1106          1331          1105
LargeGeoJSONParse                                                -0.0043         -0.0048         28292         28171         28291         28157
LargeGeoJSONTileIndex                                            -0.4434         -0.4438         26996         15026         26983         15007
LargeGeoJSONGetTile/iterations:1/repeats:9_mean                  -0.7254         -0.7254           207            57           206            57
LargeGeoJSONGetTile/iterations:1/repeats:9_median                -0.7210         -0.7211           205            57           205            57
LargeGeoJSONGetTile/iterations:1/repeats:9_stddev                -0.4379         -0.4380             6             4             6             4
LargeGeoJSONToTile                                               +0.0218         +0.0236          1202          1228          1200          1228
SingleTileIndex                                                  +0.0532         +0.0533             0             0             0             0
SingleTileGeoJSONToTile                                          -0.0675         -0.0675            12            12            12            12

// Single run with units, compare.py removes unit types
-----------------------------------------------------------------------------------------
Benchmark                                                  Time           CPU Iterations
-----------------------------------------------------------------------------------------
ParseGeoJSON                                            2275 us       2275 us        308
GenerateTileIndex                                       8501 us       8496 us         75
TraverseTilePyramid/iterations:3                        1106 ms       1105 ms          3
LargeGeoJSONParse                                      28171 us      28157 us         24
LargeGeoJSONTileIndex                                  15026 us      15007 us         49
LargeGeoJSONGetTile/iterations:1/repeats:9_mean           57 ms         57 ms          1
LargeGeoJSONGetTile/iterations:1/repeats:9_median         57 ms         57 ms          1
LargeGeoJSONGetTile/iterations:1/repeats:9_stddev          4 ms          4 ms          1
LargeGeoJSONToTile                                      1228 us       1228 us        565
SingleTileIndex                                            0 us          0 us   65396725
SingleTileGeoJSONToTile                                   12 us         12 us      61809