add pub sub client implementation with examples (#22)

* add pub sub client implementation with examples

* replace subscribed_channels list Vec with HashSet to avoid duplicates

* update Subscriber to use async-stream instead of manual Stream impl

* revert update to error handling server.rs, as #21 handles it

* remove uneeded recursion limit extension

Cargo.toml

@@ -12,6 +12,7 @@ tokio = { git = "https://github.com/tokio-rs/tokio", features = ["full"] }
 tracing = "0.1.13"
 tracing-futures = { version = "0.2.3", features = ["tokio"] }
 tracing-subscriber = "0.2.2"
+async-stream = "0.2.1"
 
 [dev-dependencies]
 # Enable test-utilities in dev mode only. This is mostly for tests.