Improve mesh channels for general-purpose use

[jstarks]

Let's make mesh channels more useful, so that we aren't tempted to reach for other channel implementations:

• mesh_channel: Combine SPSC and MPSC channels #596
• Make mesh_node optional for MPSC channel.
• Optimize oneshot channel for single-node use.
• mesh_channel: new oneshot channel implementation #613
• Remove 'static and Send bounds on channel elements.
• Add a bounded MPSC channel.
• Make bounded MPSC channels work well for RPC.
• Support channels in no_std environments (by falling back to Rc/RefCell).

[smalis-msft]

• Replace all uses of all other channels
• Tweak wording of clippy lints forbidding futures::channel in .clippy.toml