Support task-local overrides

[jam1garner]

Making this into its own issue from #33.

I have no idea how I want to do this 👍

Presumably a similar API to #35, but I have no clue what the best way to do task-locals is. Presumably this will need to be supported per executor. I imagine this will require supporting tokio and async-std separately? I know smol by itself doesn't support task-locals, but are there other executors that would need this?

I feel it goes without saying that these will be behind feature-flags.