Rollup merge of #35444 - alexcrichton:optimize-catch-unwind, r=brson

std: Optimize panic::catch_unwind slightly

The previous implementation of this function was overly conservative with
liberal usage of `Option` and `.unwrap()` which in theory never triggers. This
commit essentially removes the `Option`s in favor of unsafe implementations,
improving the code generation of the fast path for LLVM to see through what's
happening more clearly.

cc #34727