Header-only coordijk.h

[ajfriend]

This is a simple change on top of #1145. This just moves the contents of coordijk.c to a header-only coordijk.h. The ability to inline gives us more speed improvements on indexing:

| Function               |   master |   aj/header-only-coordijk |   Change |
|------------------------|----------|---------------------------|----------|
| latLngToCell           | 0.1969us |                  0.1780us |    -9.6% |
| cellToLatLng           | 0.1304us |                  0.0886us |   -32.1% |
| cellToBoundary         | 0.5790us |                  0.3897us |   -32.7% |
| directedEdgeToBoundary | 0.2994us |                  0.2004us |   -33.1% |
| vertexToLatLng         | 0.1703us |                  0.1223us |   -28.2% |

master=006ba2c73  aj/header-only-coordijk=0da8f75d8  bench=7891697

The above reflects the combined effect of both #1145 and this PR (baseline current master branch).

The effect of just this PR (baseline vec3d-core):

| Function               |   vec3d-core |   aj/header-only-coordijk |   Change |
|------------------------|--------------|---------------------------|----------|
| latLngToCell           |     0.1907us |                  0.1784us |    -6.4% |
| cellToLatLng           |     0.1061us |                  0.0885us |   -16.6% |
| cellToBoundary         |     0.4346us |                  0.3911us |   -10.0% |
| directedEdgeToBoundary |     0.2529us |                  0.2003us |   -20.8% |
| vertexToLatLng         |     0.1459us |                  0.1225us |   -16.0% |

vec3d-core=a06d1687b  aj/header-only-coordijk=98a538313  bench=7891697

[coveralls]

coverage: 99.161% (+0.1%) from 99.066% — ajfriend:aj/header-only-coordijk into uber:master