async in Traits

Currently, async fn cannot be used in traits. The reasons for this are somewhat complex, but there are plans to remove this restriction in the future.

In the meantime, however, this can be worked around using the async_trait crate from crates.io.

Note that using these trait methods will result in a heap allocation per-function-call. This is not a significant cost for the vast majority of applications, but should be considered when deciding whether to use this functionality in the public API of a low-level function that is expected to be called millions of times a second.