Feature request: Support `task_local!`

[al8n]

Currently, the smol runtime does not have something similar to tokio::task_local! and async_std::task_local!.

Reference: smol-rs/smol#329

[notgull]

Related to #84

[zeenix]

I've wished for this for so long. It will make an important API of zbus, so much better.