Add Geo Commands

This commit integrates geo commands (https://matt.sh/redis-geo) into Redis.

The only major remaining issue I'm aware of is: we need to clean up my copy/pasted zset access functions in `zset.{c,h}` with a correct zset access API.

Once we get the zset interface fixed, we can squash
relevant commits in this branch and have one nice commit
to merge into unstable.

This commit adds:
  - Geo commands
  - Tests; runnable with: ./runtest --single unit/geo
  - Geo helpers in deps/geohash-int/
  - src/geo.{c,h} and src/geojson.{c,h} implementing geo commands
  - Updated build configurations to get everything working
  - TEMPORARY: src/zset.{c,h} implementing zset score and zset
    range reading without writing to client output buffers.
  - Modified linkage of one t_zset.c function for use in zset.c