Implement synchronous channel.

[lthibault]

@evan-schott In the spirit of keeping PRs small, what do you think about merging the current state?

Here's an overview of progress so far:

• ✅ https://github.com/wetware/ww/issues/79
• ✅ Add support for multiple producers and multiple consumers
• ✅ Add support for graceful cancelation of Send and Recv
• 🏗️ Implement Peek method (+ tests)
• 🏗️ Implement Close method (+ tests)
• 🏗️ Add benchmarks for send-recv op

The last three items can go in separate PRs.

I think it would make sense for you to review the PR, taking the time to explore/test locally, and approve when you feel you've got a good understanding of how the underlying implementation works. From there, you can be responsible for implementing the next three items, and opening their associated PRs. Does that sound good?

If so, I'll create an "EPIC" issue to track the overall project.

[lthibault]

For posterity: we departed a bit from the original scope of this PR.

Synchronous channel now includes:

• send
• recv
• close

As per discussion with @evan-schott today, next step is to add unit tests for the various method-call orders that can occur on a channel.