Replace `use id = path;` with `use path as id;`

[Rendered view](https://github.com/pnkfelix/rfcs/blob/fsk-use-path-as-id/active/0000-use-path-as-id.md)
NB. this is taken directly from Haskell's [as patterns](http://en.wikibooks.org/wiki/Haskell/Pattern_matching#As-patterns), including the current binding order.