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

Summary: Change the rebinding syntax from `use ID = PATH` to `use PATH as ID`, so that paths all line up on the left side, and imported identifers are all on the right side.  Also modify `extern crate` syntax
analogously, for consistency.

[Rendered view](https://github.com/pnkfelix/rfcs/blob/fsk-use-path-as-id/active/0000-use-path-as-id.md)