Big revamp in the implmentation of labels/gotos

Added restriction that, when a label is created, there cannot be
another label with the same name visible. That allows backward goto's
to be resolved when they are read. Backward goto's get a close if
they jump out of the scope of some variable; labels get a close only
if previous goto to it jumps out of the scope of some upvalue.

lparser.h

@@ -88,6 +88,7 @@ typedef struct Labeldesc {
   int pc;  /* position in code */
   int line;  /* line where it appeared */
   lu_byte nactvar;  /* local level where it appears in current block */
+  lu_byte close;  /* goto that escapes upvalues */
 } Labeldesc;
 
 
@@ -128,6 +129,7 @@ typedef struct FuncState {
   int np;  /* number of elements in 'p' */
   int nabslineinfo;  /* number of elements in 'abslineinfo' */
   int firstlocal;  /* index of first local var (in Dyndata array) */
+  int firstlabel;  /* index of first label (in 'dyd->label->arr') */
   short nlocvars;  /* number of elements in 'f->locvars' */
   lu_byte nactvar;  /* number of active local variables */
   lu_byte nups;  /* number of upvalues */