Add 'find_indices'

src/advent/util.cr

@@ -33,6 +33,14 @@ class Array(T)
       puts
     end
   end
+
+  def find_indices
+    into = [] of T
+    each_with_index do |v, i|
+      into << i if yield v
+    end
+    into
+  end
 end
 
 struct Tuple(*T)