* ext/objspace/object_tracing.c: add new 3 methods to control tracing.
* ObjectSpace::trace_object_allocations_start
* ObjectSpace::trace_object_allocations_stop
* ObjectSpace::trace_object_allocations_clear
  And some refactoring.
* test/objspace/test_objspace.rb: add a test for new methods.
* NEWS: add a description for new methods.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43095 b2dd03c8-39d4-4d8f-98ff-823fe69b080e