Remove grpc-rs dep?

[nrc]

We depend on grpc-rs which causes a big dent in our compile time, in particular due to grpc-sys. However, the only reason we depend on grpc-rs is for some high-level types. With a bit of refactoring, we could move those into a separate crate (grpc-types) and greatly reduce our compile time.

[siddontang]

Great!!!

[gregwebs]

@nrc is this issue still relevant?

[nrc]

Yeah, I think so, though not very high priorty