Proposal: make checkpoint_if_cancelled() sync-colored and rename it accordingly

I'd like to propose that we deprecate checkpoint_if_cancelled() in favor of a synchronous function that I'm thinking of as trio.check_cancelled() (happy to take suggestions on the name, or move into hazmat). This would raise Cancelled if the current scope is cancelled, and do nothing otherwise. i.e., it is a pure cancel point, not a schedule point, not even a schedule point if we're cancelled.

This becomes possible due to some recent or pending changes:

• #474, which permits the sync-color by saying we don't need to checkpoint before raising an exception (I'm assuming Cancelled counts as an exception for the purposes of that rule)
• #901, which means the Cancelled-raising path is just raise Cancelled._init()
• #958, which makes it fast to see if we're cancelled or not

And I think would have benefits for:

• #70, by implementing efficient checkpoint() as await cancel_shielded_checkpoint(); check_cancelled()
• #606, allowing run_sync_in_worker_thread functions to call check_cancelled() occasionally without messing with portals (because it's synchronous! and if/when #958 lands I think we can make the check_cancelled() operation sufficiently thread-safe)
• iirc #658 indicated some user interest in a sync-colored raise-cancelled operation?

Possible downside: a sync-colored function called from an async function can now call check_cancelled() and wind up raising Cancelled. Previously that couldn't happen. My feeling is that "functions that do something off-the-wall might raise an exception their caller wasn't expecting" is hardly a new problem, but maybe there's some aspect of what we'd be giving up here that I'm missing.

Start by reading the proposal and the referenced issues #474, #901, #958, #70, #606, and #658, then review the five-comment thread. Determine whether checkpoint_if_cancelled() should be deprecated in favor of a synchronous check_cancelled() API, including its name, location, cancellation behavior, and thread-safety expectations.