iseq.c: struct accessors

* iseq.c (rb_method_for_self_aref, rb_method_for_self_aset): call
  accessor functions directly, not to be affected by [] and []=
  methods.  [ruby-core:66846] [Bug #10601]
* struct.c (define_aref_method, define_aset_method): ditto.
* vm_insnhelper.c (rb_vm_opt_struct_aref, rb_vm_opt_struct_aset):
  direct accessors of Struct.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48864 b2dd03c8-39d4-4d8f-98ff-823fe69b080e