refactor: reintroduce RangeList

spec/range_list_spec.cr

@@ -8,7 +8,7 @@ describe TimeCost::RangeList do
 
 	describe ".new" do
 		it "can be created without arguments" do
-			assert_instance_of TimeCost::RangeList, list
+			list.should be_a(TimeCost::RangeList)
 		end
 	end
 

src/timecost/range_list.cr

@@ -4,7 +4,10 @@ module TimeCost
       @ranges = [] of Range
     end
 
-    def add(range : Range)
+    def add(commit : Commit) : RangeList
+    end
+
+    def add(range : Range) : RangeList
       merged = false
       merged_range = nil
 
@@ -34,7 +37,7 @@ module TimeCost
       end
     end
 
-    def sum
+    def sum : Float
       result = 0
       @ranges.each do |r|
         result += r.diff