Reduce julia inference time for ExceptionUnwrapping.jl

[NHDaly]

1. Use @nospecialize to prevent julia from codegen'ing specializations
   for every possible combination of wrapped exception types, since the
   resultant code will be essentially the same, and we don't mind
   paying the runtime cost for the (rare!) exception handling
   codepaths.
2. Use inferencebarriers to prevent julia from attempting to infer
   these functions until they're actually called (at the price of one
   extra dispatch during exception handling).
3. Convert to explicitly untyped while loops instead of recursion, since
   julia would have attempted to statically resolve the recursion, and
   we saw it hit a stack overflow during inference in production.

[jakebolewski]

👍 , looks good to me.