New semantics for the integer 'for' loop

The numerical 'for' loop over integers now uses a precomputed counter
to control its number of iteractions. This change eliminates several
weird cases caused by overflows (wrap-around) in the control variable.
(It also ensures that every integer loop halts.)

Also, the special opcodes for the usual case of step==1 were removed.
(The new code is already somewhat complex for the usual case,
but efficient.)

lopnames.h

@@ -84,8 +84,6 @@ static const char *const opnames[] = {
   "RETURN",
   "RETURN0",
   "RETURN1",
-  "FORLOOP1",
-  "FORPREP1",
   "FORLOOP",
   "FORPREP",
   "TFORPREP",