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.

ARMeilleure/Translation/SsaConstruction.cs

@@ -140,7 +140,7 @@ namespace ARMeilleure.Translation
                     }
                 }
 
-                Array.Clear(localDefs, 0, localDefs.Length);
+                Array.Clear(localDefs);
             }
         }