Add distinct connections over Io (#18)

Real world servers often use concurrency for processing requests.
Protocols also lean on message ordering per connection, ie TCP. Neither
of these features were available with the current `Io` implementation.

This change overlays connection support on top of `Io` by multiplexing
through an actor. A simple TCP-like state machine is used to initiate and
accept connections.

Cargo.toml

@@ -17,11 +17,14 @@ keywords = ["Deterministic", "Simulation", "Testing"]
 categories = ["asynchronous", "network-programming", "simulation"]
 
 [dependencies]
+futures = "0.3"
 indexmap = "1.9"
 rand = { version = "0.8.5", features = ["small_rng"] }
 rand_distr = "0.4.3"
 scoped-tls = "1.0.0"
 serde = { version = "1.0.143", features = ["derive"] }
 serde_json = "1.0.83"
-tokio = { version = "1.19.2", features = ["rt", "sync", "test-util", "time"] }
+tokio = { version = "1.19.2", features = ["full"] }
+tokio-stream = "0.1.9"
 tokio-test = { version = "0.4.2" }
+unicycle = "0.9.2"