move rust-mode.el to src/etc/emacs/

[graydon]

It's currently living over in marijn's "temporary" repo https://github.com/marijnh/rust-mode but has long since become the only emacs mode that can speak rust, so..

[brson]

Need to also change the mention on the getting started wiki page when it moves.

[marijnh]

Done.