More uniformity in code generation for 'for' loops

Added new instruction 'OP_TFORPREP' to prepare a generic for loop.
Currently it is equivalent to a jump (but with a format 'iABx',
similar to other for-loop preparing instructions), but soon it will
be the place to create upvalues for closing loop states.

ljumptab.h

@@ -96,6 +96,7 @@ static void *disptab[] = {
 &&L_OP_FORPREP1,
 &&L_OP_FORLOOP,
 &&L_OP_FORPREP,
+&&L_OP_TFORPREP,
 &&L_OP_TFORCALL,
 &&L_OP_TFORLOOP,
 &&L_OP_SETLIST,