Another way to compile goto's

The compilation of a goto or a label just create an entry and generate
boilerplate code for the gotos. As we don't know yet whether it needs a
CLOSE, we code a jump followed by a CLOSE, which is then dead code.

When a block ends (and then we know for sure whether there are variables
that need to be closed), we check the goto's against the labels of that
block. When closing a goto against a label, if it needs a CLOSE, the
compiler swaps the order of the jump and the CLOSE, making the CLOSE
active.

lparser.h

@@ -112,7 +112,7 @@ typedef struct Labeldesc {
   int pc;  /* position in code */
   int line;  /* line where it appeared */
   lu_byte nactvar;  /* number of active variables in that position */
-  lu_byte close;  /* goto that escapes upvalues */
+  lu_byte close;  /* true for goto that escapes upvalues */
 } Labeldesc;