1. Wherever the `buf` field of a `Formatter` was used, the `Formatter` is used
instead.
2. The usage of `write_fmt` is minimized as much as possible, the `write!` macro
is preferred wherever possible.
3. Usage of `fmt::write` is minimized, favoring the `write!` macro instead.
markdown files.
This means that
# Foo
## Bar
# Baz
### Qux
## Quz
Gets a TOC like
1 Foo
1.1 Bar
2 Baz
2.0.1 Qux
2.1 Quz
This functionality is only used when rendering a single markdown file,
never on an individual module, although it could very feasibly be
extended to allow modules to opt-in to a table of contents (std::fmt
comes to mind).
