investigate the use of a low-overhead async engine

[refacktor]

Investigate possible use of tokio::runtime::Builder::new_current_thread() or smol.

Reddit thread: https://www.reddit.com/r/rust/comments/1h40cdd/comment/lzxc0q1

Goals

• Introduce lightweight async as an optional feature
• Validate options and feasibility (tokio may conflict with no_main; not sure about smol)
• Quantify and document the actual overhead involved, to help decide when to enable the feature.

[refacktor]

Because of the way the Lambda Runtime interface works, this will only impact async processing within the handler. Handler invocation is always one-at-a-time synchronous.