New kind of expression VKSTR

String literal expressions have their own kind VKSTR, instead of the
generic VK. This allows strings to "cross" functions without entering
their constant tables (e.g., if they are used only by some nested
function).

lparser.h

@@ -30,7 +30,9 @@ typedef enum {
   VFALSE,  /* constant false */
   VK,  /* constant in 'k'; info = index of constant in 'k' */
   VKFLT,  /* floating constant; nval = numerical float value */
-  VKINT,  /* integer constant; nval = numerical integer value */
+  VKINT,  /* integer constant; ival = numerical integer value */
+  VKSTR,  /* string constant; strval = TString address;
+             (string is fixed by the lexer) */
   VNONRELOC,  /* expression has its value in a fixed register;
                  info = result register */
   VLOCAL,  /* local variable; var.ridx = local register;
@@ -67,6 +69,7 @@ typedef struct expdesc {
   union {
     lua_Integer ival;    /* for VKINT */
     lua_Number nval;  /* for VKFLT */
+    TString *strval;  /* for VKSTR */
     int info;  /* for generic use */
     struct {  /* for indexed variables */
       short idx;  /* index (R or "long" K) */