debug utility macro rp() (rp_m()) and bp() are introduced.
* rp(obj) shows obj information w/o any side-effect to STDERR.
* rp_m(m, obj) is similar to rp(obj), but show m before.
* bp() is alias of ruby_debug_breakpoint(), which is registered
  as a breakpoint in run.gdb (used by `make gdb` or make gdb-ruby`).