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.)

ljumptab.h

@@ -99,8 +99,6 @@ static void *disptab[NUM_OPCODES] = {
 &&L_OP_RETURN,
 &&L_OP_RETURN0,
 &&L_OP_RETURN1,
-&&L_OP_FORLOOP1,
-&&L_OP_FORPREP1,
 &&L_OP_FORLOOP,
 &&L_OP_FORPREP,
 &&L_OP_TFORPREP,