* Parser understands the contextual keyword `stream` to indicate whether
an input or response is a stream of messages.
* Methods like `stream stream()` and `stream stream stream()` are
handled properly (the first is a unary method and the second is a
server stream).
* JSON AST augmented with streaming cardnality.
* C# codegen generates the right code for streaming methods.
* Added streaming and generics C# gRPC examples.
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