Async zero-cost claim

[turalcar]

async-book/src/01_getting_started/02_why_async.md

Line 52 in ed022fc

- **Async is zero-cost** in Rust, which means that you only pay for what you use.

That is false. RawWaker very explicitly requires a dynamic dispatch, i. e. it's not zero-cost compared to, say, mio crate.