gc.c: add GC.latest_gc_info()

* gc.c (struct rb_objspace): rename internal last_collection_flags to
  latest_gc_info
* gc.c (gc_latest_collection_info): add GC.latest_gc_info() with similar
  behavior to GC.stat()
* gc.c (rb_gc_latest_gc_info): new c-api for above
* gc.c (gc_stat_internal): remove :last_collection_flags from GC.stat
* gc.c (gc_profile_decode_flags): remove GC::Profiler.decode_flags
* include/ruby/intern.h (rb_gc_latest_gc_info): export new c-api
* test/ruby/test_gc.rb (class TestGc): test for new behavior
* NEWS: note about new api

* gc.c (gc_stat_internal): raise TypeError on wrong type
* gc.c (gc_stat): fix error message

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44008 b2dd03c8-39d4-4d8f-98ff-823fe69b080e