add DISTANCE_UNITS

spec/geo_spec.cr

@@ -0,0 +1,27 @@
+require "./spec_helper"
+
+describe Geo do
+  context "DISTANCE_UNITS" do
+    it do
+      distance_units = {
+        :centimeters,
+        :centimetres,
+        :degrees,
+        :feet,
+        :inches,
+        :kilometers,
+        :kilometres,
+        :meters,
+        :metres,
+        :miles,
+        :millimeters,
+        :millimetres,
+        :nautical_miles,
+        :radians,
+        :yards,
+      }
+
+      Geo::DISTANCE_UNITS.should eq(distance_units)
+    end
+  end
+end

src/geo.cr

@@ -1,10 +1,14 @@
 require "convex_hull"
 require "geohash"
 require "geo_bearing"
+require "haversine"
 require "./geo/utils"
 require "./geo/coord"
 require "./geo/polygon"
+require "./geo/distance"
 
 module Geo
   VERSION = {{ `shards version #{__DIR__}`.chomp.stringify }}
+
+  DISTANCE_UNITS = Haversine::FACTORS.keys
 end

src/geo/distance.cr

@@ -1,5 +1,3 @@
-require "haversine"
-
 module Geo
   struct Coord
     # Calculates distance to `other`.