Some minor cleanups and optimizations (#4174)

* Replace Array.Clear(x, 0, x.Length) with Array.Clear(x)

* Use DateTime.UnixEpoch field

* Replace SHA256.ComputeHash calls with static SHA256.HashData call

More performant and avoids the need to initialize a SHA256 instance.

Ryujinx.HLE/HOS/Services/Ro/IRoInterface.cs

@@ -142,14 +142,9 @@ namespace Ryujinx.HLE.HOS.Services.Ro
 
             _owner.CpuMemory.Read(nroAddress, nroData);
 
-            byte[] nroHash = null;
-
             MemoryStream stream = new MemoryStream(nroData);
 
-            using (SHA256 hasher = SHA256.Create())
-            {
-                nroHash = hasher.ComputeHash(stream);
-            }
+            byte[] nroHash = SHA256.HashData(stream);
 
             if (!IsNroHashPresent(nroHash))
             {