Rename stuff to keep up with new 'checkpoint' nomenclature

[njsmith]

assert_yields → assert_checkpoints
assert_no_yields → assert_no_checkpoints just deprecate without replacement
yield_briefly → checkpoint
yield_briefly_no_cancel → checkpoint_never_cancelled, schedule_point, checkpoint_shielded, ...? checkpoint_ignore_cancelled? checkpoint_with_cancel_shield? never_cancelled_checkpoint? cancel_shielded_checkpoint
yield_if_cancelled → checkpoint_if_cancelled? if_cancelled_checkpoint?

yield_indefinitely can stay the same, or maybe block_indefinitely, block_task_indefinitely? Naw, let's make it yield_indefinitely → wait_task_rescheduled, for parallelism with the other wait functions.

[njsmith]

Will probably do this right after landing the giant ssl branch of doom.

[njsmith]

Actually, we should probably just deprecate trio.testing.assert_no_yields entirely, with no replacement. I've never found a use for it, and can't think of any either...