Debugging/profiling features will be located.
* vm_trace.c: expose C-level TracePoint APIs.
  Note that they are experimental.
* vm_trace.c, include/ruby/debug.h: rename `rb_hook_flag_t'
  to `rb_event_hook_flag_t'.
  Macro names `RUBY_HOOK_FLAG_*' are also renamed to
  `RUBY_EVENT_HOOK_FLAG_*'.
* debug.h, vm_debug.h: rename debug.h to vm_debug.h.
* common.mk: ditto.
* debug.c, main.c, vm_core.h: ditto.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37765 b2dd03c8-39d4-4d8f-98ff-823fe69b080e