Tables have a 'lastfree' information only when needed

Only tables with some minimum number of entries in their hash part
have a 'lastfree' field, kept in a header before the node vector.

lmem.h

@@ -63,6 +63,8 @@
 
 #define luaM_newobject(L,tag,s)	luaM_malloc_(L, (s), tag)
 
+#define luaM_newblock(L, size)	luaM_newvector(L, size, char)
+
 #define luaM_growvector(L,v,nelems,size,t,limit,e) \
 	((v)=cast(t *, luaM_growaux_(L,v,nelems,&(size),sizeof(t), \
                          luaM_limitN(limit,t),e)))