Add Crypto::Bcrypt::Password#==

Previously, it used `Reference` equality, which compares identity rather
than value.

spec/std/crypto/bcrypt/password_spec.cr

@@ -78,4 +78,13 @@ describe "Crypto::Bcrypt::Password" do
       (password2y.verify "secret").should be_true
     end
   end
+
+  describe "#==" do
+    it "is equal if the string representations are equal" do
+      canonical = Crypto::Bcrypt::Password.create("secret", 4)
+      challenge = Crypto::Bcrypt::Password.new(canonical.to_s)
+
+      canonical.should eq challenge
+    end
+  end
 end

src/crypto/bcrypt/password.cr

@@ -83,4 +83,6 @@ class Crypto::Bcrypt::Password
   def inspect(io : IO) : Nil
     to_s(io)
   end
+
+  def_equals_and_hash
 end