vm.c: add RubyVM.stat for accessing cache serials

* vm.c (ruby_vm_stat): add RubyVM.stat() for access to internal cache
  counters. this methods behaves like GC.stat, accepting an optional
  hash or symbol argument. [Bug #9190] [ruby-core:58750]
* test/ruby/test_rubyvm.rb: test for new method

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44062 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* properties.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44063 b2dd03c8-39d4-4d8f-98ff-823fe69b080e