Rollup merge of #125392 - workingjubilee:unwind-a-problem-in-context,…

… r=Amanieu

Wrap Context.ext in AssertUnwindSafe

Fixes rust-lang/rust#125193

Alternative to rust-lang/rust#125377

Relevant to rust-lang/rust#123392

I believe this approach is justifiable due to the fact that this function is unstable API and we have been considering trying to dispose of the notion of "unwind safety". Making a more long-term decision should be considered carefully as part of stabilizing `fn ext`, if ever.

r? `@Amanieu`