refactor(lua): use Arena when converting from lua stack to API args

and for return value of nlua_exec/nlua_call_ref, as this uses
the same family of functions.

NB: the handling of luaref:s is a bit of a mess.
add api_luarefs_free_XX functions as a stop-gap as refactoring
luarefs is a can of worms for another PR:s.

as a minor feature/bug-fix, nvim_buf_call and nvim_win_call now preserves
arbitrary return values.

src/nvim/lua/xdiff.c

@@ -209,7 +209,8 @@ static bool check_xdiff_opt(ObjectType actType, ObjectType expType, const char *
 static NluaXdiffMode process_xdl_diff_opts(lua_State *lstate, xdemitconf_t *cfg, xpparam_t *params,
                                            int64_t *linematch, Error *err)
 {
-  const DictionaryOf(LuaRef) opts = nlua_pop_Dictionary(lstate, true, err);
+  // TODO: this is very much a keydict..
+  const DictionaryOf(LuaRef) opts = nlua_pop_Dictionary(lstate, true, NULL, err);
 
   NluaXdiffMode mode = kNluaXdiffModeUnified;