feat(streams): Abstract stream connections

[grypez]

Streams are asynchronous remote i/o interfaces. By wrapping them around connections, we get a uniform interface for passing messages to targets regardless of their sync/async//local/remote nature. Using the stream envelope kit, we can constrain the kinds of messages passed either way both at compile time and runtime. We can also pipe streams to one another, allowing us to connect targets across paths proxied through different connection types without writing a specific interface for every pair of connections.

We should wrap the background <--> offscreen connection in a stream, and have the kernel operate on an input stream, too.

[grypez]

Excessively scoped. Relevant parts reassigned to #97.