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).

testes/code.lua

@@ -409,5 +409,22 @@ checkequal(function () return 6 and true or nil end,
            function () return k6 and kTrue or kNil end)
 
 
+do   -- string constants
+  local function f1 ()
+    local <const> k = "00000000000000000000000000000000000000000000000000"
+    return function ()
+             return function () return k end
+           end
+  end
+
+  local f2 = f1()
+  local f3 = f2()
+  assert(f3() == string.rep("0", 50))
+  checkK(f3, f3())
+  -- string is not needed by other functions
+  assert(T.listk(f1)[1] == nil)
+  assert(T.listk(f2)[1] == nil)
+end
+
 print 'OK'