lazy_async_promise: Simple primitives to manage tokio and egui

This crate currently only features two simple primitives for getting computation time off the main thread using tokio:

• LazyVecPromise for a vector-backed storage which can be partially displayed
• LazyValuePromise for a single value future

As the name suggests both of them are lazily evaluated and nothing happens until polled for the first time.

Example-usage of this crate with a small egui/eframe blog-reader can be found here