Try to find a way to flip the order of `mitigate` and `within`

[Jon Pretty]

Currently, the "mitigators" have to be defined before the body, which is not intuitive. It's not clear what would be the best way to do this, though.

[Jon Pretty]

One possible way may be to write it as a try/catch block within a mitigate block, like this:

mitigate:
  try
    // some action
  catch
    case SomeError() => NewError()

but it's not clear whether the block would be accepted before the macro rewrites it.

[Jon Pretty]

mitigate/within is effectively deprecated in favor of given ... fixes ... now.