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.

lopcodes.h

@@ -282,6 +282,7 @@ OP_FORLOOP,/*	A Bx	R(A)+=R(A+2);
 			if R(A) <?= R(A+1) then { pc-=Bx; R(A+3)=R(A) }	*/
 OP_FORPREP,/*	A Bx	R(A)-=R(A+2); pc+=Bx				*/
 
+OP_TFORPREP,/*	A Bx	create upvalue A; pc+=Bx			*/
 OP_TFORCALL,/*	A C	R(A+3), ... ,R(A+2+C) := R(A)(R(A+1), R(A+2));	*/
 OP_TFORLOOP,/*	A Bx	if R(A+1) ~= nil then { R(A)=R(A+1); pc -= Bx }	*/