We've switched to dashes

The shebang is thrown away so this has no end-user impact

According to the [RFC](https://rust-lang.github.io/rfcs/3503-frontmatter.html):

> Opens with 3+ dashes followed by 0+ whitespace,
> an optional term (one or more characters excluding whitespace and commas),
> 0+ whitespace, and a newline

From the [RFC](https://rust-lang.github.io/rfcs/3503-frontmatter.html)

> When parsing Rust source, after stripping the shebang (#!), rustc will strip the frontmatter:
>
> - May include 0+ blank lines (whitespace + newline)

The question is what is a non-newline whitespace and what is a newline.
Looking at the [Reference](https://doc.rust-lang.org/reference/whitespace.html),
the answer is "unsure", particularly because the Rust language doesn't
generally try to distinguish them.
I kept things basic for now and we can revisit later.