Add MaybeNull attribute on ValueOrNull

Author: thomaslevesque

src/Hamlet/NullableAttributes.cs

@@ -0,0 +1,7 @@
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class MaybeNullAttribute : Attribute
+    { }
+}

src/Hamlet/Option.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Hamlet
 {
@@ -148,6 +149,7 @@ public static U Match<T, U>(this Option<T> option, Func<T, U> some, Func<U> none
         /// <typeparam name="T">The type of the option's value.</typeparam>
         /// <param name="option">The option to get the value from.</param>
         /// <returns>The option's value if it's <c>Some</c>, or <c>null</c> if it's <c>None</c>.</returns>
+        [return: MaybeNull]
         public static T ValueOrNull<T>(this Option<T> option)
             where T : class?
         {