Rollup merge of #80736 - KodrAus:feat/lazy-resolve, r=dtolnay

use Once instead of Mutex to manage capture resolution

For #78299

This allows us to return borrows of the captured backtrace frames that are tied to a borrow of the Backtrace itself, instead of to some short-lived Mutex guard.

We could alternatively share `&Mutex<Capture>`s and lock on-demand, but then we could potentially forget to call `resolve()` before working with the capture. It also makes it semantically clearer what synchronization is needed on the capture.

cc `@seanchen1991` `@rust-lang/project-error-handling`

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