Rollup merge of #100455 - xfix:backtrace-ref-unwind-safe, r=dtolnay

Implement RefUnwindSafe for Backtrace

Backtrace doesn't have visible mutable state.

See also https://internals.rust-lang.org/t/should-backtrace-be-refunwindsafe/17169?u=xfix

Author: matthiaskrgr