refactor: Change type for city.population to bigint

Refs: #92

Author: ghusta

scripts/world.sql

@@ -14,7 +14,7 @@ CREATE TABLE city (
     name text NOT NULL,
     country_code character(3) NOT NULL,
     district text NOT NULL,
-    population integer NOT NULL,
+    population bigint NOT NULL,
     local_name text NULL
 );