Redesign and harden string interning.

Up to 40% faster on hash-intensive benchmarks.
With some ideas from Sokolov Yura.

src/lj_str.h

@@ -20,8 +20,12 @@ LJ_FUNC int lj_str_haspattern(GCstr *s);
 LJ_FUNC void lj_str_resize(lua_State *L, MSize newmask);
 LJ_FUNCA GCstr *lj_str_new(lua_State *L, const char *str, size_t len);
 LJ_FUNC void LJ_FASTCALL lj_str_free(global_State *g, GCstr *s);
+LJ_FUNC void LJ_FASTCALL lj_str_init(lua_State *L);
+#define lj_str_freetab(g) \
+  (lj_mem_freevec(g, g->str.tab, g->str.mask+1, GCRef))
 
 #define lj_str_newz(L, s)	(lj_str_new(L, s, strlen(s)))
 #define lj_str_newlit(L, s)	(lj_str_new(L, "" s, sizeof(s)-1))
+#define lj_str_size(len)	(sizeof(GCstr) + (((len)+4) & ~(MSize)3))
 
 #endif