Make `location: location!()` less painful to use

snafu::Location is a struct we can include in our error types. The Location struct records the line at which the error is created. This is very helpful for debugging. However, there are a few drawbacks:

1. We need to call location: location!() at every error init site. This is tedious and annoying.
2. While the SomethingSnafu types generated can automatically record locations, there are complications with it as well.
3. It's unclear to new comers to the repo what the heck is a Snafu
4. the location captured in async code is not correct without special tricks: https://docs.rs/snafu/latest/snafu/struct.Location.html#asynchronous-code

Idea: Maybe we could write a proc macro that scans for all Error init sites and inject location: location!() calls everywhere.

Start from the issue's proposal to design a proc macro that scans error initialization sites and injects location calls. First investigate how Snafu error types and async locations currently work; done would require a concrete, agreed-upon approach that addresses both repetitive call sites and async correctness.