write an indexer that uses built-in postgres gist index

[tracyhenry]

It is confirmed now that postgres has built-in geometry types and GiST indexes. So we should write a new indexer that uses it, test if it works fine, and then possibly get rid of PostGIS, which is a pain to install.

https://www.postgresql.org/docs/9.4/functions-geometry.html
https://www.citusdata.com/blog/2017/10/17/tour-of-postgres-index-types/
https://medium.com/@Alibaba_Cloud/principles-and-applications-of-the-index-types-supported-by-postgresql-481f59bab67d