Implement Set#add? method

spec/std/set_spec.cr

@@ -40,6 +40,20 @@ describe "Set" do
     end
   end
 
+  describe "add?" do
+    it "returns true when object is not in the set" do
+      set = Set(Int32).new
+      set.add?(1).should be_true
+    end
+
+    it "returns false when object is in the set" do
+      set = Set(Int32).new
+      set.add?(1).should be_true
+      set.includes?(1).should be_true
+      set.add?(1).should be_false
+    end
+  end
+
   describe "delete" do
     it "deletes an object" do
       set = Set{1, 2, 3}

src/set.cr

@@ -71,6 +71,19 @@ struct Set(T)
     self
   end
 
+  # Adds *object* to the set and returns `true` on success
+  # and `false` if the value was already in the set.
+  #
+  # ```
+  # s = Set{1, 5}
+  # s.add? 8 # => true
+  # s.add? 8 # => false
+  # ```
+  def add?(object : T)
+    # TODO: optimize the hash lookup call
+    !!(add(object) unless includes?(object))
+  end
+
   # Adds `#each` element of *elems* to the set and returns `self`.
   #
   # ```