Per discussion in
https://github.com/status-im/nim-chronos/pull/251#issuecomment-1559233139,
`async: (parameters..)` is introduced as a way to customize the async
transformation instead of relying on separate keywords (like
asyncraises).
Two parameters are available as of now:
`raises`: controls the exception effect tracking
`raw`: disables body transformation
Parameters are added to `async` as a tuple allowing more params to be
added easily in the future:
```nim:
proc f() {.async: (name: value, ...).}`
```
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