* vm_trace.c (symbol2event_flag): add secret feature.
  add a_call/a_return events.
  a_call is call | b_call | c_call, and same as a_return.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43645 b2dd03c8-39d4-4d8f-98ff-823fe69b080e