Idea: ToBumpString

I find I end up writing

```rust
let x = bumpalo::format!(in cx.bump, "{}", var).into_bump_str();
```

a lot, so my suggestion is to add

```rust
trait ToBumpString: Display {
fn to_bump_str<'a>(&self, arena: &'a Bump) -> &'a str {
bumpalo::format!(in arena, "{}", var).into_bump_str();
}
```

matching `ToString` (except that we want a `&str`).

Search the bumpalo crate for the format!, into_bump_str, Bump, Display, and ToString-related APIs mentioned in the issue, then inspect how public traits are organized. Done means the proposed ToBumpString behavior is integrated as a coherent public API, returns an arena-backed &str, and has coverage for formatting a Display value.

Written by the indexing model from the issue text.