Add convenience methods for Duration (#571)

* dur

* tests

---------

Author: bitfaster

BitFaster.Caching.UnitTests/DurationTests.cs

@@ -0,0 +1,23 @@
+using System;
+using FluentAssertions;
+using Xunit;
+
+namespace BitFaster.Caching.UnitTests
+{
+    public class DurationTests
+    {
+        [Fact]
+        public void RoundTripHours()
+        {
+            var d = Duration.FromHours(2);
+            d.ToTimeSpan().Should().BeCloseTo(TimeSpan.FromHours(2), TimeSpan.FromMilliseconds(100));
+        }
+
+        [Fact]
+        public void RoundTripDays()
+        {
+            var d = Duration.FromDays(2);
+            d.ToTimeSpan().Should().BeCloseTo(TimeSpan.FromDays(2), TimeSpan.FromMilliseconds(100));
+        }
+    }
+}

BitFaster.Caching/Duration.cs

@@ -94,15 +94,35 @@ public static Duration FromSeconds(double value)
         }
 
         /// <summary>
-        /// Returns a Duration that represents a specified number of milliseconds.
+        /// Returns a Duration that represents a specified number of minutes.
         /// </summary>
-        /// <param name="value">A number of milliseconds</param>
+        /// <param name="value">A number of minutes</param>
         /// <returns></returns>
         public static Duration FromMinutes(double value)
         {
             return FromTimeSpan(TimeSpan.FromMinutes(value));
         }
 
+        /// <summary>
+        /// Returns a Duration that represents a specified number of hours.
+        /// </summary>
+        /// <param name="value">A number of hours</param>
+        /// <returns></returns>
+        public static Duration FromHours(double value)
+        {
+            return FromTimeSpan(TimeSpan.FromHours(value));
+        }
+
+        /// <summary>
+        /// Returns a Duration that represents a specified number of days.
+        /// </summary>
+        /// <param name="value">A number of days</param>
+        /// <returns></returns>
+        public static Duration FromDays(double value)
+        {
+            return FromTimeSpan(TimeSpan.FromDays(value));
+        }
+
         /// <summary>
         /// Adds two specified Duration instances.
         /// </summary>