context.Context friendly API

https://github.com/matryer/vice/blob/660007f4486b289078664b182d1ecd2a22b29b02/transport.go#L24

Done() in context.Context returns a `<-chan struct{}`, which is not compatible with vice.Transport.

```
*Transport does not implement vice.Transport (wrong type for Done method)
have Done() <-chan struct {}
want Done() chan struct {}
```

Start at transport.go line 24, the linked definition of vice.Transport, and compare it with context.Context's Done method. Confirm the intended compatible API from the reported compiler error, then run the repository's Go tests or build to verify that the interface mismatch is resolved.

Written by the indexing model from the issue text.