Implement single sender, single receiver synchronous channel

[evan-schott]

Create implementation in wetware/ww/pkg/csp/basic.go by following the flow chart below. Use go channel to signal between receiver and sender, and lock to control access to sent value.

[lthibault]

Use go channel to signal between receiver and sender

Note that this is not a hard requirement. You might find it simpler to use a mutex to protect some fields, e.g.

type syncChan struct {
    mu sync.Mutex
    sendrReady, recvrReady bool
    value capnp.Ptr  // the value being transferred from sender to recver
}

(Then again, maybe the channel is actually easier. Your call.)

[lthibault]

https://hackmd.io/@fCsHyW7yR3C5lGQFbh9KdQ/BkiAsWeTs

☝️ POC implementation using sync.Cond + important notes about correctness.

[lthibault]

Closing per #88