* ext/objspace/objspace.c: support ObjectSpace.trace_object_allocations.
  Read the following test to know HOWTO.
  This feature is a sample of RUBY_INTERNAL_EVENT.
* test/objspace/test_objspace.rb: add a test.
* ext/objspace/object_tracing.c: ditto.
* gc.c (rb_gc_count): add. THis function returns GC count.
* internal.h: add decl. of rb_gc_count(). Same as `GC.count'.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40957 b2dd03c8-39d4-4d8f-98ff-823fe69b080e