RegExp.re or similar seems nice:
```
let re = RegExp.re("x")`
(\d{3}-)? # area code (optional)
\d{3}- # prefix
\d{4} # line number
`;
```
But it seems like previous proposals of this want escaping which doesn't seem ideal for this purpose. Do we need both `RegExp.re` and `RegExp.escapedRe`?