Add evaluation classes for CalDateTime

* Added `CalDateTimeEvaluator`
* Added lightweight `CalDateTimeZoned` which is returned from `CalDateTimeEvaluator` methods
* Updated existing `CalDateTime` comparion and arithmethic related methods to use `CalDateTimeEvaluator`
* Added `Microsoft.Bcl.HashCode` for simplified HashCode generation with `netstandard2.x`

Author: axunonb

Ical.Net.Tests/CalDateTimeTests.cs

@@ -149,7 +149,7 @@ public string ToStringTests(CalDateTime calDateTime, string format, IFormatProvi
     public static IEnumerable ToStringTestCases()
     {
         yield return new TestCaseData(new CalDateTime(2024, 8, 30, 10, 30, 0, tzId: "Pacific/Auckland"), "O", null)
-            .Returns("2024-08-30T10:30:00.0000000+12:00 Pacific/Auckland")
+            .Returns("2024-08-30T10:30:00.0000000 Pacific/Auckland")
             .SetName("Date and time with 'O' format arg, default culture");
 
         yield return new TestCaseData(new CalDateTime(2024, 8, 30), "O", null)
@@ -158,7 +158,7 @@ public static IEnumerable ToStringTestCases()
 
         yield return new TestCaseData(new CalDateTime(2024, 8, 30, 10, 30, 0, tzId: "Pacific/Auckland"), "O",
                 CultureInfo.GetCultureInfo("fr-FR"))
-            .Returns("2024-08-30T10:30:00.0000000+12:00 Pacific/Auckland")
+            .Returns("2024-08-30T10:30:00.0000000 Pacific/Auckland")
             .SetName("Date and time with 'O' format arg, French culture");
 
         yield return new TestCaseData(new CalDateTime(2024, 8, 30, 10, 30, 0, tzId: "Pacific/Auckland"),