Add solution for day 3.

common.cr

@@ -0,0 +1,22 @@
+module Enumerable(T)
+  def count_each(others)
+    count_map = {} of T => Int32
+    each do |other|
+      count_map[other] = (count_map[other]? || 0) + 1
+    end
+    return count_map
+  end
+end
+
+class Rectangle
+  def initialize(@x : Int32, @y : Int32, @width : Int32, @height : Int32)
+  end
+
+  def each_coord(&block)
+    (@x...(@x + @width)).each do |x|
+      (@y...(@y + @height)).each do |y|
+        yield x, y
+      end
+    end
+  end
+end