Alter logic in Time.leap_year? for performance

spec/std/time/time_spec.cr

@@ -745,6 +745,32 @@ describe Time do
     Time.days_in_year(1990).should eq(365)
   end
 
+  describe ".leap_year?" do
+    it "knows that 400-year centuries are leap years" do
+      {1600, 2000, 2400}.each do |year|
+        Time.leap_year?(year).should be_true
+      end
+    end
+
+    it "knows that 100-year centuries are normal years" do
+      {1700, 1800, 1900, 2100, 2200, 2300}.each do |year|
+        Time.leap_year?(year).should be_false
+      end
+    end
+
+    it "knows that typical non-century leap years are divisibly by 4" do
+      {1968, 1972, 2004, 2020}.each do |year|
+        Time.leap_year?(year).should be_true
+      end
+    end
+
+    it "knows years *not* divisible by 4 are normal" do
+      {1965, 1999, 2001, 2018, 2019, 2021, 2099, 2101}.each do |year|
+        Time.leap_year?(year).should be_false
+      end
+    end
+  end
+
   typeof(Time.now.year)
   typeof(1.minute.from_now.year)
   typeof(1.minute.ago.year)

src/time.cr

@@ -477,7 +477,7 @@ struct Time
       raise ArgumentError.new "Invalid year"
     end
 
-    (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)
+    year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
   end
 
   def inspect(io : IO)