feat(channel): introduce new channel component

[azjezz]

Inspired by async_std::channel module, The Channel component provides asynchronous channels for communication between different component of an application.

e.g:

use Psl\Async;
use Psl\Channel;

/**
 * @var Channel\ReceiverInterface<string> $receiver
 * @var Channel\SenderInterface<string> $sender
 */
[$receiver, $sender] = Channel\bounded(2);

Async\Scheduler::defer(function() use($sender) {
  Async\sleep(1); // sleep for a second.

  $sender->send('hello');
});

$message = $receiver->receive(); // 'hello'

[coveralls]

Pull Request Test Coverage Report for Build 1496391738

• 92 of 92 (100.0%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 1493967235:	0.0%
Covered Lines:	3297
Relevant Lines:	3297

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💛 - Coveralls