Refactor "RandomSubset" internals for DRY, removing closure

This is a squashed merge of PR #1049.

---------

Co-authored-by: Atif Aziz <code@raboof.com>

MoreLinq/RandomSubset.cs

@@ -61,26 +61,25 @@ public static IEnumerable<T> RandomSubset<T>(this IEnumerable<T> source, int sub
             if (source == null) throw new ArgumentNullException(nameof(source));
             if (subsetSize < 0) throw new ArgumentOutOfRangeException(nameof(subsetSize));
 
-            return RandomSubsetImpl(source, rand, seq => (seq.ToArray(), subsetSize));
+            return RandomSubsetImpl(source, rand, subsetSize);
         }
 
-        static IEnumerable<T> RandomSubsetImpl<T>(IEnumerable<T> source, Random rand, Func<IEnumerable<T>, (T[], int)> seeder)
+        static IEnumerable<T> RandomSubsetImpl<T>(IEnumerable<T> source, Random rand, int? subsetSize)
         {
             // The simplest and most efficient way to return a random subset is to perform
             // an in-place, partial Fisher-Yates shuffle of the sequence. While we could do
             // a full shuffle, it would be wasteful in the cases where subsetSize is shorter
             // than the length of the sequence.
             // See: http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle
 
-            var (array, subsetSize) = seeder(source);
+            var array = source.ToArray();
+            subsetSize ??= array.Length;
 
             if (array.Length < subsetSize)
             {
-#pragma warning disable CA2208 // Instantiate argument exceptions correctly
-                               // TODO Throw InvalidOperationException instead?
+                // TODO Throw InvalidOperationException instead?
                 throw new ArgumentOutOfRangeException(nameof(subsetSize),
                     "Subset size must be less than or equal to the source length.");
-#pragma warning restore CA2208 // Instantiate argument exceptions correctly
             }
 
             var m = 0;                // keeps track of count items shuffled

MoreLinq/Shuffle.cs

@@ -19,7 +19,6 @@ namespace MoreLinq
 {
     using System;
     using System.Collections.Generic;
-    using System.Linq;
 
     public static partial class MoreEnumerable
     {
@@ -70,11 +69,7 @@ public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> source, Random rand)
             if (source == null) throw new ArgumentNullException(nameof(source));
             if (rand == null) throw new ArgumentNullException(nameof(rand));
 
-            return RandomSubsetImpl(source, rand, seq =>
-            {
-                var array = seq.ToArray();
-                return (array, array.Length);
-            });
+            return RandomSubsetImpl(source, rand, subsetSize: null);
         }
     }
 }