symbols instead of IDs

* encoding.c (rb_enc_get_index): deal with symbols instead of IDs
  to get rid of inadvertent pin-downs.

* enum.c (chunk_ii): ditto.

* enumerator.c (append_method): ditto.

* iseq.c (iseq_load): ditto.

* marshal.c (w_symbol, r_symlink, r_symreal, r_symbol): ditto.

* signal.c (trap_handler): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46988 b2dd03c8-39d4-4d8f-98ff-823fe69b080e