feature: add bson_to_lightuserdata api. see #2074 (#2075)

lualib-src/lua-bson.c

@@ -327,6 +327,7 @@ append_one(struct bson *bs, lua_State *L, const char *key, size_t sz, int depth)
 	case LUA_TNUMBER:
 		append_number(bs, L, key, sz);
 		break;
+	case LUA_TLIGHTUSERDATA:
 	case LUA_TUSERDATA: {
 		append_key(bs, L, BSON_DOCUMENT, key, sz);
 		int32_t * doc = (int32_t*)lua_touserdata(L,-1);
@@ -978,6 +979,13 @@ lencode(lua_State *L) {
 	return 1;
 }
 
+static int
+lto_lightuserdata(lua_State *L) {
+	void *p = lua_touserdata(L, 1);
+	lua_pushlightuserdata(L, p);
+	return 1;
+}
+
 static int
 encode_bson_byorder(lua_State *L) {
 	int n = lua_gettop(L);
@@ -1340,6 +1348,7 @@ luaopen_bson(lua_State *L) {
 		{ "objectid", lobjectid },
 		{ "int64", lint64 },
 		{ "decode", ldecode },
+		{ "to_lightuserdata", lto_lightuserdata },
 		{ NULL,  NULL },
 	};