gc.c: expose GC.stat() to C-api via rb_gc_stat()

* include/ruby/intern.h: add rb_gc_stat() for access to GC.stat
  variables from c-api
* gc.c (rb_gc_stat): new c-api method. accepts either VALUE hash like
  GC.stat, or VALUE symbol key and returns size_t directly. the second
  form is useful to avoid allocations, i.e. for usage inside
  INTERNAL_EVENT_GC tracepoints.
* gc.c (gc_stat): add GC.stat(:key) to return single value instead of hash
* gc.c (gc_stat_internal): helper method to retrieve single or all stat values
* test/ruby/test_gc.rb (class TestGc): test for new behavior
* NEWS: note about this new api

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44005 b2dd03c8-39d4-4d8f-98ff-823fe69b080e