Feature/async await

[plwai]

• [x] Update crates

• [x] Update core implementation

• [x] Update tests

• [x] Update example

• [x] Benchmark comparison

close #31

[jk-gan]

I will review it in this weekend

[jk-gan]

any reason for switching from tokio to async-std?

[plwai]

async-std api is cleaner and easier compared with tokio.