runtime: add tracing span for block_on futures

[seanmonstar]

This adds a tracing span to the future in block_on, when the unstable tracing support is enabled. While not technically a spawn, we still call it a "task spawn" with the kind set to block_on.

Unfortunately, it cannot collect the #[tokio::main] task, since even though it is instrumented, the task is entered before we can call console_subscriber::init(). But all manual usage of block_on is collected.

Closes tokio-rs/console#106

[hawkw]

Unfortunately, it cannot collect the #[tokio::main] task, since even though it is instrumented, the task is entered before we can call console_subscriber::init(). But all manual usage of block_on is collected.

it would be really nice if this were not the case...but, hmm, fixing that seems like a big pile of work.