Remove 'static bound from Async trait alias

[soareschen]

Summary

This PR removes the 'static bound from the Async trait, which was originally an alias to Send + Sync + 'static.

The Async trait is important for CGP code to more easily define abstract types that can be used more easily within async functions. Currently, it is necessary for async functions in traits to include at least the Send bound in the returned Future, in order for it to be usable in async executors such as tokio::spawn. This implies that almost all abstract types need to have at least the Send + Sync trait bounds to be included.

Although CGP makes it possible to add these bounds at the implementation site, it would quickly become overly tedious if we need to do this for almost every abstract type. As a result, CGP encourages abstract types to be define with Send + Sync bounds for them to be usable inside async code. Originally, the 'static bound was also included, as it was also common for async code to also require 'static bound when spawning async tasks.

The Async trait is introduced as a helper trait, so that users can use it instead of writing Send + Sync everywhere. Originally, the 'static bound was also included, as it is common for async code to also require 'static when spawning async tasks. However, in practice, the use is infrequent enough that we don't need to include that inside the default trait bound for all abstract types.

Furthermore, the inclusion of 'static inside Async by default makes it more challenging to instantiate abstract types with concrete types that contain non-static lifetimes. As the use of CGP grows, we want to avoid the default from making it almost impossible to use non-static concrete types with CGP. As a result, it is best to remove this before CGP gains wider adoption.

On the other hand, the default inclusion of Send + Sync is almost a necessary evil with the current state of async Rust. However, this may soon change when Return Type Notation (RTN) gets stabilized in Rust in the near future in rust-lang/rust#138424. Once that is stabilized, the Async trait itself can entirely be deprecated or removed.

However, until RTN is stabilized, we still need Async trait with only the Send + Sync bounds. It is also unclear when RTN will actually gets stabilized, and whether we will be able to completely remove Async by then. Hence, at the very least, it is better to remove 'static from Async in case if Async lingers longer than we expect.