Add Enumerable(T)#to_set(& : T -> U) : Set(U) forall U

spec/std/enumerable_spec.cr

@@ -130,6 +130,20 @@ describe "Enumerable" do
     end
   end
 
+  describe "#to_set" do
+    context "without block" do
+      it "creates a Set from the unique elements of the collection" do
+        {1, 1, 2, 3}.to_set.should eq Set{1, 2, 3}
+      end
+    end
+
+    context "with block" do
+      it "creates a Set from running the block against the collection's elements" do
+        {1, 2, 3, 4, 5}.to_set { |i| i // 2 }.should eq Set{0, 1, 2}
+      end
+    end
+  end
+
   describe "chunk" do
     it "works" do
       [1].chunk { true }.to_a.should eq [{true, [1]}]

src/set.cr

@@ -486,4 +486,14 @@ module Enumerable
   def to_set : Set(T)
     Set.new(self)
   end
+
+  # Returns a new `Set` with the unique results of running *block* against each
+  # element of the enumerable.
+  def to_set(&block : T -> U) : Set(U) forall U
+    set = Set(U).new
+    each do |elem|
+      set << yield elem
+    end
+    set
+  end
 end