FFI: Add ffi.gc() function for finalization of cdata objects.

src/lj_ctype.h

@@ -158,6 +158,7 @@ typedef struct CTState {
   MSize sizetab;	/* Size of C type table. */
   lua_State *L;		/* Lua state (needed for errors and allocations). */
   global_State *g;	/* Global state. */
+  GCtab *finalizer;	/* Map of cdata to finalizer. */
   CTypeID1 hash[CTHASH_SIZE];  /* Hash anchors for C type table. */
 } CTState;
 
@@ -428,7 +429,7 @@ LJ_FUNC CTInfo lj_ctype_info(CTState *cts, CTypeID id, CTSize *szp);
 LJ_FUNC GCstr *lj_ctype_repr(lua_State *L, CTypeID id, GCstr *name);
 LJ_FUNC GCstr *lj_ctype_repr_int64(lua_State *L, uint64_t n, int isunsigned);
 LJ_FUNC GCstr *lj_ctype_repr_complex(lua_State *L, void *sp, CTSize size);
-LJ_FUNC void lj_ctype_init(lua_State *L);
+LJ_FUNC CTState *lj_ctype_init(lua_State *L);
 LJ_FUNC void lj_ctype_freestate(global_State *g);
 
 #endif