This improves the bm_vm2_bighash benchmark significantly by
removing branches during insert, but slows down anything
requiring iteration with the more complex loop termination
checking.

Speedup ratio of 1.10 - 1.20 is typical for the vm2_bighash
benchmark.

v3 - st_head calculates list_head address in two steps
     to avoid a bug in old gcc 4.4 (Debian 4.4.7-2)
     bug which incorrectly warned with:
     warning: dereferencing pointer ‘({anonymous})’ does break
     strict-aliasing rules

* include/ruby/st.h (struct st_table): hide struct list_head
* st.c (struct st_table_entry): adjust struct
  (head, tail): remove shortcut macros
  (st_head): new wrapper function
  (st_init_table_with_size): adjust to new struct and API
  (st_clear): ditto
  (add_direct): ditto
  (unpack_entries): ditto
  (rehash): ditto
  (st_copy): ditto
  (remove_entry): ditto
  (st_shift): ditto
  (st_foreach_check): ditto
  (st_foreach): ditto
  (get_keys): ditto
  (get_values): ditto
  (st_values_check): ditto
  (st_reverse_foreach_check): ditto (unused)
  (st_reverse_foreach): ditto (unused)
  [ruby-core:69726] [Misc #10278]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51064 b2dd03c8-39d4-4d8f-98ff-823fe69b080e