Refactor: Remove some unnecessary uses of memcpy

- Also, remove unnecessary "static" in gnucash-style.c

The second one in guid.cpp is UB:

libgnucash/engine/guid.cpp:137:5: warning: undefined behavior, source object type 'const gnc::GUID' is not TriviallyCopyable [bugprone-undefined-memory-manipulation]
        memcpy (&target, &source, sizeof (GncGUID));
        ^

libgnucash/backend/sql/gnc-slots-sql.cpp

@@ -160,7 +160,7 @@ _retrieve_guid_ (gpointer pObject,  gpointer pValue)
     g_return_if_fail (pObject != NULL);
     g_return_if_fail (pValue != NULL);
 
-    memcpy (pGuid, guid, sizeof (GncGUID));
+    *pGuid = *guid;
 }
 
 /* Special column table because we need to be able to access the table by

libgnucash/backend/sql/gnc-sql-column-table-entry.cpp

@@ -649,7 +649,7 @@ _retrieve_guid_ (gpointer pObject,  gpointer pValue)
     g_return_if_fail (pObject != NULL);
     g_return_if_fail (pValue != NULL);
 
-    memcpy (pGuid, guid, sizeof (GncGUID));
+    *pGuid = *guid;
 }
 
 // Table to retrieve just the guid