* funcobject.c (func_repr): don't call getstringvalue(None) for anonymous
  functions.
* bltinmodule.c: removed lambda (which is now a built-in function);
  removed implied lambda for string arg to filter/map/reduce.
* Grammar, graminit.[ch], compile.[ch]: replaced lambda as built-in
  function by lambda as grammar entity: instead of "lambda('x: x+1')" you
  write "lambda x: x+1".
* Xtmodule.c (checkargdict): return 0, not NULL, for error.