Header-only coordijk.h v2

[ajfriend]

Same as #1148, but cleaner git history.

Benchmarks comparing this PR to the current master, run with https://github.com/ajfriend/h3-index-bench:

                      Comparison (median of 300 samples)
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━┓
┃ Function                   ┃   master ┃ aj/header-only-coordijk-v2 ┃ Change ┃
┡━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━┩
│ latLngToCell               │ 0.1877us │                   0.1754us │  -6.6% │
│ cellToLatLng               │ 0.1024us │                   0.0894us │ -12.7% │
│ cellToBoundary             │ 0.4271us │                   0.3909us │  -8.5% │
│ directedEdgeToBoundary     │ 0.2417us │                   0.2009us │ -16.9% │
│ vertexToLatLng             │ 0.1434us │                   0.1192us │ -16.9% │
└────────────────────────────┴──────────┴────────────────────────────┴────────┘
     master=46ef6b953  aj/header-only-coordijk-v2=4cb98bf59  bench=7891697

[coveralls]

coverage: 99.162%. remained the same — ajfriend:aj/header-only-coordijk-v2 into uber:master