encoding.h: constify rb_encoding

* include/ruby/encoding.h: constify `rb_encoding` itself, not only
  arguments.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46322 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
encoding.c: move ruby_encoding_index stuff

* encoding.c: move `ruby_encoding_index` stuff from
  include/ruby/encoding.h to hide the extra field.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46323 b2dd03c8-39d4-4d8f-98ff-823fe69b080e