Change in the handling of 'L->top' when calling metamethods

Instead of updating 'L->top' in every place that may call a
metamethod, the metamethod functions themselves (luaT_trybinTM and
luaT_callorderTM) correct the top. (When calling metamethods from
the C API, however, the callers must preserve 'L->top'.)

testes/coroutine.lua

@@ -809,7 +809,7 @@ assert(run(function ()
 -- tests for coroutine API
 if T==nil then
   (Message or print)('\n >>> testC not active: skipping coroutine API tests <<<\n')
-  return
+  print "OK"; return
 end
 
 print('testing coroutine API')