This recovers and improves performance of Marshal.dump/load on
Time objects compared to when we implemented generic ivars
entirely using st_table.

This also recovers some performance on other generic ivar objects,
but does not bring bring Marshal.dump/load performance up to
previous speeds.

benchmark results:
minimum results in each 10 measurements.
Execution time (sec)
		name    trunk   geniv   after
marshal_dump_flo        0.343   0.334   0.335
marshal_dump_load_geniv 0.487   0.527   0.495
marshal_dump_load_time  1.262   1.401   1.257

Speedup ratio: compare with the result of `trunk' (greater is better)
		name    geniv   after
marshal_dump_flo        1.026   1.023
marshal_dump_load_geniv 0.925   0.985
marshal_dump_load_time  0.901   1.004

* include/ruby/intern.h (rb_generic_ivar_table): deprecate
* internal.h (rb_attr_delete): declare
* marshal.c (has_ivars): use rb_ivar_foreach
  (w_ivar): ditto
  (w_object): update for new interface
* time.c (time_mload): use rb_attr_delete
* variable.c (generic_ivar_delete): implement
  (rb_ivar_delete): ditto
  (rb_attr_delete): ditto
  [ruby-core:69323] [Feature #11170]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50680 b2dd03c8-39d4-4d8f-98ff-823fe69b080e