Move main() to its own file

main() is formatted with clang-format too and we're also adding
a simple build infrastructure for making a working test executable.
Fix license formatting
Style clean up (manually)

Fixes:
  - Easier to read offsets in blocks of actions
  - Initialize i in for loop instead of before for loop
  - Make functions void if they never return failure
  - Move variable declarations to top of open braces
  - Remove unneeded stdio.h header (we extracted main() elsewhere)
Fix format specifier for unsigned long long

If other platforms complain, we can use system-defined type specifiers.
Add geohash_helper

This is geohash_helper.{cpp,hpp} converted to C from
https://github.com/yinqiwen/ardb/blob/d42503/src/geo/geohash_helper.cpp
https://github.com/yinqiwen/ardb/blob/d42503/src/geo/geohash_helper.hpp

It compiles cleanly, but is untested so far.
Rename lon to long and fix some interfaces

We don't need pointers as arguments everywhere because:
  - a.) we aren't modifying the parameters
  - b.) the structres aren't that big