Feature request / discuss: allow errors to have multiple causes

I've run into multiple situations now where X can only succeed if A, B, and C all succeed, but executing each of A/B/C happens independently, so I'd like to be able to do something like the below example.

Using contexts doesn't really make sense here, because A/B/C failing are all independent of each other, they don't cause one another, and contexts are meant to be used for "error 1 happened because of error 2"

Have you ever considered doing this? Do you know anyone who's made a crate like this? This doesn't appear to be something that anyhow/thiserror/snafu support, but it feels like something that could reasonably fit into anyhow.

```
// this is pseudo-code, meant to illustrate the _sentiment_ of what i want, not the actual API
fn x() -> anyhow::Result<()> {
let mut errs = vec![];
match a() {
Ok(_) => ...,
Err(e) => { errs.push(e); },
}
match b() {
Ok(_) => ...,
Err(e) => { errs.push(e); },
}
match c() {
Ok(_) => ...,
Err(e) => { errs.push(e); },
}
if errs.empty() {
Ok(())
} else {
Err(anyhow::Error::sum(errs))
}
```

Start by reviewing anyhow's existing Error and context APIs, then compare how anyhow, thiserror, and snafu represent related errors. The issue does not name files or tests; done would require a settled API and behavior for collecting and reporting independent causes.

Written by the indexing model from the issue text.