* parse.y: rename symbols::sym_id to symbols::str_id.
  This table is not {Symbol => ID} table, but
  {String => ID} table.
* parse.y (lookup_sym_id): also rename lookup_sym_id() to
  lookup_str_id() because key is not Symbol, but String.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46696 b2dd03c8-39d4-4d8f-98ff-823fe69b080e