Make musig2 private nonce loadable

NBitcoin.Tests/Secp256k1Tests.cs

@@ -4114,9 +4114,7 @@ public void musig_tweak_vectors()
 		private MusigPrivNonce ToMusigPrivNonce(string hex)
 		{
 			var b = Encoders.Hex.DecodeData(hex);
-			return new MusigPrivNonce(ECPrivKey.Create(b.AsSpan().Slice(0, 32)),
-									  ECPrivKey.Create(b.AsSpan().Slice(32, 32)),
-									  ECPubKey.Create(b.AsSpan().Slice(64, 33)));
+			return MusigPrivNonce.Load(b);
 		}
 
 		[Fact]

NBitcoin/Secp256k1/Musig/MusigPrivNonce.cs

@@ -17,7 +17,6 @@ class MusigPrivNonce : IDisposable
 	{
 		readonly static RandomNumberGenerator rand = RandomNumberGenerator.Create();
 
-
 		/// <summary>
 		/// This function derives a secret nonce that will be required for signing and
 		/// creates a private nonce whose public part intended to be sent to other signers.
@@ -175,6 +174,16 @@ internal MusigPrivNonce(ECPrivKey k1, ECPrivKey k2, ECPubKey pk)
 			this.pk = pk;
 		}
 
+		public static MusigPrivNonce Load(Span<byte> bytes)
+		{
+			if(bytes.Length != 97)
+				throw new FormatException("Invalid private nonce, expected 97 bytes");
+			var k1 = ECPrivKey.Create(bytes[..32]);
+			var k2 = ECPrivKey.Create(bytes[32..64]);
+			var pk = ECPubKey.Create(bytes[64..]);
+			return new MusigPrivNonce(k1, k2, pk);
+		}
+
 		public void Dispose()
 		{
 			k1.Dispose();