Add debug counters to RubyVM.stat

[chrisseaton]

For example I use this like this

def foo
  14
end

before = RubyVM.stat

10.times do
  foo
end

after = RubyVM.stat

pp after.map { |k, v| [k, v - (before[k] || 0)] }.to_h.select { |k, v| v > 0 }

{:constant_cache_misses=>1,
 :mc_inline_hit=>9,
 :mc_inline_miss_klass=>3,
 :mc_inline_miss_diff=>3,
 :mc_search=>2,
 :mc_search_super=>3,
 :cc_new=>2,
 :cc_found_in_ccs=>1,
 :cc_not_found_in_ccs=>2,
 :ccs_not_found=>2,
 :ccf_general=>2,
 :ccf_iseq_setup=>1,
 :ccf_iseq_fix=>9,
 :ccf_cfunc=>1,
 :ccf_cfunc_with_frame=>2,
 :frame_push=>22,
 :frame_push_method=>10,
 :frame_push_block=>10,
 :frame_push_cfunc=>2,
 :frame_R2R=>10,
 :frame_R2C=>2,
 :frame_C2R=>10,
 :lvar_set=>1,
 :lvar_set_slowpath=>1,
 :obj_newobj=>319,
 :artable_hint_miss=>1,
 :artable_hint_notfound=>8,
 :heap_xmalloc=>41,
 :heap_xfree=>9,
 :theap_alloc=>1,
 :vm_sync_lock_enter=>262,
 :vm_sync_lock_enter_nb=>1}

[chrisseaton]

I think all previous review points have been addressed.

[k0kubun]

I wonder if it's enough to separately define it as RubyVM.debug_counters like other USE_DEBUG_COUNTER-only methods, but looking at the use case, I think he wanted to compare all VM stats together and this would have been more useful for him. Given that it's USE_DEBUG_COUNTER-only and we could move it at any time, I'll merge this as is. Thank you, Chris.