* benchmark/bm_hash_shift.rb: add benchmark for Hash#shift

* hash.c (rb_hash_shift): use st_shift if hash is not being iterated to
  delete element without iterating the whole hash.

* hash.c (shift_i): remove function

* include/ruby/st.h (st_shift): add st_shift function

* st.c (st_shift): ditto


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@41652 b2dd03c8-39d4-4d8f-98ff-823fe69b080e