🔨 Add static Random instance to RandomExtensions

To prevent creating new instances on every method call, library now
refers to a single static Random instance, when a random operation
is needed.

X10D/src/CharExtensions.cs

@@ -18,7 +18,7 @@
         /// <returns>Returns a <see cref="string"/> containing <paramref name="length"/> characters.</returns>
         public static string Random(this char[] chars, int length)
         {
-            return chars.Random(length, new Random());
+            return chars.Random(length, RandomExtensions.Random);
         }
 
         /// <summary>
@@ -47,7 +47,7 @@
         /// <returns>Returns a <see cref="string"/> containing <paramref name="length"/> characters.</returns>
         public static string Random(this IEnumerable<char> chars, int length)
         {
-            return chars.Random(length, new Random());
+            return chars.Random(length, RandomExtensions.Random);
         }
 
         /// <summary>

X10D/src/RandomExtensions.cs

@@ -9,22 +9,36 @@
     /// </summary>
     public static class RandomExtensions
     {
+        /// <summary>
+        /// Gets the <see cref="System.Random"/> instance to which other extension methods may refer, when one is
+        /// needed but not provided.
+        /// </summary>
+        internal static Random Random { get; } = new Random();
+
         /// <summary>
         /// Returns either <see langword="true"/> or <see langword="false"/> based on <paramref name="random"/>'s next
         /// generation.
         /// </summary>
-        /// <param name="random">The <see cref="Random"/> instance.</param>
+        /// <param name="random">The <see cref="System.Random"/> instance.</param>
+        /// <returns>Returns <see langword="true"/> or <see langword="false"/> depending on the return value
+        /// from <see cref="System.Random.Next(int)"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="random"/> is <see langword="null"/>.</exception>
         public static bool CoinToss(this Random random)
         {
+            if (random is null)
+            {
+                throw new ArgumentNullException(nameof(random));
+            }
+
             return random.Next(2) == 0;
         }
 
         /// <summary>
-        /// Returns a random element from <paramref name="source"/> using the <see cref="Random"/> instance.
+        /// Returns a random element from <paramref name="source"/> using the <see cref="System.Random"/> instance.
         /// </summary>
         /// <typeparam name="T">The collection type.</typeparam>
-        /// <param name="random">The <see cref="Random"/> instance.</param>
+        /// <param name="random">The <see cref="System.Random"/> instance.</param>
-        /// <param name="source">The collection to draw from.</param>
+        /// <param name="source">The collection from which to draw.</param>
         /// <returns>Returns a random element of type <see cref="T"/> from <paramref name="source"/>.</returns>
         public static T OneOf<T>(this Random random, params T[] source)
         {
@@ -32,11 +46,11 @@
         }
 
         /// <summary>
-        /// Returns a random element from <paramref name="source"/> using the <see cref="Random"/> instance.
+        /// Returns a random element from <paramref name="source"/> using the <see cref="System.Random"/> instance.
         /// </summary>
         /// <typeparam name="T">The collection type.</typeparam>
-        /// <param name="random">The <see cref="Random"/> instance.</param>
+        /// <param name="random">The <see cref="System.Random"/> instance.</param>
-        /// <param name="source">The collection to draw from.</param>
+        /// <param name="source">The collection from which to draw.</param>
         /// <returns>Returns a random element of type <see cref="T"/> from <paramref name="source"/>.</returns>
         public static T OneOf<T>(this Random random, IList<T> source)
         {

X10D/src/StringExtensions.cs

@@ -105,7 +105,7 @@
         /// <returns>Returns a <see cref="string"/> containing <paramref name="length"/> characters.</returns>
         public static string Random(this string str, int length)
         {
-            return str.Random(length, new Random());
+            return str.Random(length, RandomExtensions.Random);
         }
 
         /// <summary>
@@ -146,7 +146,7 @@
         /// <returns>Returns a <see cref="string"/> containing the characters in <paramref name="str"/>, rearranged.</returns>
         public static string Shuffle(this string str)
         {
-            return str.Shuffle(new Random());
+            return str.Shuffle(RandomExtensions.Random);
         }
 
         /// <summary>