Allow tref to pass through encode/2

There are times when a `tref` may be passed to Erlang/Elixir, and then
we want to return it as-is (or modified) to the emulator. Currently,
this will fail when Luerl tries to re-encode the value.

Instead, we should just pass through these values as is

src/luerl_new.erl

@@ -395,8 +395,12 @@ encode({M,F,A}, St) when is_atom(M) and is_atom(F) ->
     {#erl_mfa{m=M,f=F,a=A}, St};
 encode({userdata,Data}, St) ->
     luerl_heap:alloc_userdata(Data, St);
+% Table refs should not be re-encoded
+encode(#tref{}=T, St) ->
+    {T, St};
 encode(_, _) -> error(badarg).                  %Can't encode anything else
 
+
 %% decode_list([LuerlTerm], State) -> [Term].
 %% decode(LuerlTerm, State) -> Term.
 %%  In decode we track of which tables we have seen to detect