Move setting forward index code to grn_obj_set_value()
Move setting forward index code to grn_obj_set_value()

Now, we can set forward index value by grn_obj_set_value().

Supporting inverted index for forward index isn't implemented yet. It
is the next task.

cosmo0920/groonga,redfigure/groonga,groonga/groonga,redfigure/groonga,hiroyuki-sato/groonga,groonga/groonga,groonga/groonga,naoa/groonga,redfigure/groonga,kenhys/groonga,redfigure/groonga,groonga/groonga,hiroyuki-sato/groonga,naoa/groonga,hiroyuki-sato/groonga,komainu8/groonga,myokoym/groonga,groonga/groonga,myokoym/groonga,naoa/groonga,kenhys/groonga,hiroyuki-sato/groonga,myokoym/groonga,cosmo0920/groonga,groonga/groonga,kenhys/groonga,myokoym/groonga,myokoym/groonga,redfigure/groonga,hiroyuki-sato/groonga,myokoym/groonga,myokoym/groonga,komainu8/groonga,hiroyuki-sato/groonga,myokoym/groonga,naoa/groonga,cosmo0920/groonga,komainu8/groonga,cosmo0920/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,hiroyuki-sato/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,komainu8/groonga,redfigure/groonga,naoa/groonga,redfigure/groonga,kenhys/groonga,komainu8/groonga,cosmo0920/groonga,cosmo0920/groonga,cosmo0920/groonga,komainu8/groonga,redfigure/groonga,kenhys/groonga,naoa/groonga,hiroyuki-sato/groonga,kenhys/groonga,cosmo0920/groonga,naoa/groonga,groonga/groonga