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)
… except for the minor matter that the binding operator in match expressions has the identifier on the *left*, not the *right*. (Yeah, this has always struck me as back-to-front and I wonder whether we should swap it.)