Replace the internal sha512 implementation with .Net built in sha512 (#390)

* Replace the internal sha512 implementation with .Net built in sha512

* Add IDisposable to Sha512 wrapper

---------

Co-authored-by: Niklas Petersen <niklasfp@users.noreply.github.com>

Author: niklasfp

src/NATS.Client.Core/NaCl/Internal/Array16.cs

@@ -74,7 +74,7 @@ public static bool crypto_sign_verify(
             if (GroupOperations.ge_frombytes_negate_vartime(out A, pk, pkoffset) != 0)
                 return false;
 
-            var hasher = new Sha512();
+            using var hasher = new Sha512();
             hasher.Update(sig, sigoffset, 32);
             hasher.Update(pk, pkoffset, 32);
             hasher.Update(m, moffset, mlen);

src/NATS.Client.Core/NaCl/Internal/Array8.cs

@@ -26,7 +26,7 @@ public static void crypto_sign(
 		{
 			byte[] az, r, hram;
 			GroupElementP3 R;
-		    var hasher = new Sha512();
+		    using var hasher = new Sha512();
 			{
                 hasher.Update(sk, skoffset, 32);
 			    az = hasher.Finalize();