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.

lopnames.h

@@ -81,6 +81,7 @@ static const char *const opnames[] = {
   "FORPREP1",
   "FORLOOP",
   "FORPREP",
+  "TFORPREP",
   "TFORCALL",
   "TFORLOOP",
   "SETLIST",