Use `module_name.rs` instead of `module_name/mod.rs`.
Pros:
- It's easier to find the module file (especially for modules with lots of
submodules like `types`)
- Module file name is not 'magical'
- Make `net` module public
- Move `Bot::download_file{,_stream}` methods to a new `Download` trait
- Add `download_forward` macro to forward `Download` impls (priv)
- Impl `Download` for all bot adaptors & the `Bot` itself
- Change return type of `download_file_stream` — return `Stream<Result<Bytes>>``,
instead of `Future<Result<Stream<Result<Bytes>>>>``
- Add `api_url` param to standalone versions of `download_file{,_stream}`
- Make `net::{TELEGRAM_API_URL, download_file{,_stream}}` pub
- Small documentation changes
