upgrade lambda_runtime crate dependencies & add async handler fn support

[hanabi1224]

This PR includes below changes, please let me know ur feedback, thanks!

• Add async handler fn support
• Make tokio an optional dependency behind feature runtime (default on)
• Upgrade tokio to 1.14
• Upgrade http to 0.2
• Upgrade lambda_runtime to 0.4
• Upgrade to rust 2021

The API change in this PR should keep backward compatibility. All style changes come from cargo fmt

Resolves https://github.com/vercel-community/rust/issues/46 https://github.com/vercel-community/rust/issues/20

[frencojobs]

Could normal and async stuff be in separate files (e.g sync.rs and async.rs), since they're pretty much distinct from what I see in the implementation?

Also, why do we need this?

# Cargo.toml
[dev-dependencies]
anyhow = "1"
tokio = "1.12"

[ngryman]

Async should really be a first-class citizen. Is there anything we could do to move this forward?

[hanabi1224]

It will be great if this PR can get reviewed soon, before it's merged and published, ppl can point the crate to my fork

# Cargo.toml
vercel_lambda = {git = "https://github.com/hanabi1224/rust", branch = "update_lambda_runtime_deps"}

[ArthurianX]

Is this abandoned ?