With these fixes, `transports`/`asyncsync` correctly propagate and document their raises information - generally, most transport functions (send etc) raise `TransportError` and `CancelledError` - `closeWait` is special in that it generally doesn't fail.
This PR introduces the syntax `Future[void].Raises([types])` to create the `InternalRaisesFuture` type with the correct encoding for the types - this allows it to be used in user code while retaining the possibility to change the internal representation down the line.
* introduce raising constraints on stream callbacks - these constraints now give a warning when called with a callback that can raise exceptions (raising callbacks would crash
* fix fail and its tests, which wasn't always given a good generic match
* work around nim bugs related to macro expansion of generic types
* make sure transports raise only `TransportError`-derived exceptions (and `CancelledError`)
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