Oops, this should've been posted to -core :x

I started working on this, but ran into problems because we're current
using rb_id_serial_t which discards scoping bits (so it seems '@' is
ignored, causing conflicts for internal ivs which don't have '@')

http://80x24.org/spew/20151111201126.22805-1-e%4080x24.org/raw

Haven't had more time to revisit, but figured I post about it
here in case I don't get around to it before 2.3.0-rc1