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.)

testes/db.lua

@@ -162,7 +162,7 @@ test([[for i,v in pairs{'a','b'} do
 end
 ]], {1,2,1,2,1,3})
 
-test([[for i=1,4 do a=1 end]], {1,1,1,1,1})
+test([[for i=1,4 do a=1 end]], {1,1,1,1}, true)
 
 
 do   -- testing line info/trace with large gaps in source