H3

Added
- Added h3_distance function.

Changed
- Updated the core library to v3.1.0.

Added
- Added Direction enum, replacing int and defined constants (77)
Fixed
- Ensured unused memory is cleared for pentagon children. (84)
- Fixed compiler warnings in `h3ToGeoHier` and `h3ToGeoBoundaryHier`. (90)
- Fixed a segfault in `h3SetToLinkedGeo` (94)
Changed
- Warnings are not errors by default. (90)

Added
- Generator for the faceCenterPoint table (67)
- Generator for number of indexes per resolution (69)
- Added CMake `WARNINGS_AS_ERRORS` option, default on, for Clang and GCC (70)
- Added CMake `TEST_WRAPPER` setting when `WRAP_VALGRIND` is on (66)
Changed
- Moved Vec3d structure to `vec3d.h` (67)
- Added input validation when getting the origin or destination of an edge (74)

Changed
- Changed signature of internal function h3NeighborRotations.
- Do not require support for `hexRange`, `hexRanges`, or `hexRangeDistances` in the bindings. These functions may be deprecated in the public API in future releases; consumers should use `kRing` and `kRingDistances` instead.
- Performance improvement in core indexing logic for `geoToH3` (thanks wewei!)

Fixed
- Fixed duplicate vertex in h3ToGeoBoundary for certain class III hexagons (46)
Added
- Release guide

Added
- Assertions when allocating memory on the heap.
Changed
- Most internal memory allocations happen on the stack instead of the heap.