The content slides all use `fn main`, with the exception of the testing
segment. But with this change, where it makes sense exercises use tests
instead, and not both tests and `fn main`.
A small change in `book.js` supports running tests when a code sample
does not have `fn main` but does have `#[test]`, so these work
naturally.
Fixes#1581.
The dprint formatter is a flexible system which will use sandboxed
WebAssembly formatters to format our code (mostly: it calls out to
`rustfmt` for Rust code).
A particularly interesting feature is that dprint can format Rust code
blocks in the Markdown files. However, before we turn that on, we need
to have a way to normalize the Markdown text as it is extracted[1].
That is so that the word put into the translations is kept after the
reformatting.
[1]: https://github.com/google/mdbook-i18n-helpers/issues/19
