refactor: replace sizeof(a)/sizeof(a[0]) by std::size (C++17)

Removes the macro ARRAYLEN and also substitutes all other uses of the same
"sizeof(a)/sizeof(a[0])" pattern by std::size, available since C++17.

src/test/sighash_tests.cpp

@@ -88,7 +88,7 @@ void static RandomScript(CScript &script) {
     script = CScript();
     int ops = (InsecureRandRange(10));
     for (int i=0; i<ops; i++)
-        script << oplist[InsecureRandRange(sizeof(oplist)/sizeof(oplist[0]))];
+        script << oplist[InsecureRandRange(std::size(oplist))];
 }
 
 void static RandomTransaction(CMutableTransaction &tx, bool fSingle) {