Add shr method

Src/Autarkysoft.Bitcoin/Cryptography/EllipticCurve/Scalar8x32.cs

@@ -979,6 +979,31 @@ private static Scalar8x32 Reduce(in Scalar8x32 r, uint overflow)
         }
 
 
+        /// <summary>
+        /// Shift this scalar right by the given amount strictly between 0 and 16.
+        /// </summary>
+        /// <param name="shift">Shift must be between 0 and 16</param>
+        /// <param name="ret">Low bits that were shifted off</param>
+        /// <returns>Shifted scalar</returns>
+        public unsafe Scalar8x32 Shr16(int shift, out uint ret)
+        {
+            Debug.Assert(shift > 0);
+            Debug.Assert(shift < 16);
+
+            ret = b0 & ((1U << shift) - 1);
+            uint r0 = (b0 >> shift) + (b1 << (32 - shift));
+            uint r1 = (b1 >> shift) + (b2 << (32 - shift));
+            uint r2 = (b2 >> shift) + (b3 << (32 - shift));
+            uint r3 = (b3 >> shift) + (b4 << (32 - shift));
+            uint r4 = (b4 >> shift) + (b5 << (32 - shift));
+            uint r5 = (b5 >> shift) + (b6 << (32 - shift));
+            uint r6 = (b6 >> shift) + (b7 << (32 - shift));
+            uint r7 = (b7 >> shift);
+
+            return new Scalar8x32(r0, r1, r2, r3, r4, r5, r6, r7);
+        }
+
+
         public static unsafe Scalar8x32 MulShiftVar(in Scalar8x32 a, in Scalar8x32 b, int shift)
         {
             Debug.Assert(shift >= 256);