The attribute is applied everywhere it can be across the codebase and is
the newly preferred method for launching an application. This commit
also makes '#[rocket::main]` stricter by warning when it is applied to
functions other than 'main'.
* Update 'tokio', 'tokio-rustls', and 'hyper'.
* Remove unused dependencies on some `futures-*` crates.
* Rework 'spawn_on', which is now 'serve'.
* Simplify Ctrl-C handling.
Use I/O traits and types from 'tokio-io' as much as possible.
A few adapters only exist in futures-io-preview and use
futures-tokio-compat as a bridge for now.