improve coroutine cancel semantics

[andrewrk]

• Rename cancel to cancelsuspend. It's only for canceling coroutines which have used the suspend keyword, and they must be in the suspend state, and not running (at least not accessing the coroutine frame).

• Add cancelasync which is used for canceling an async function call. This is thread-safe and can be called at any time, even if the coroutine is running. It atomically sets a flag, and then any time the async function would start suspending - this includes doing an await - it instead performs a cancelsuspend on itself.

• When a coroutine is destroyed via cancelsuspend (always at a suspend point), it runs the defer and errdefer expressions in scope, and then either does cancelsuspend on the awaiter, or if there is no awaiter yet, sets a flag which the awaiter will discover and perform a cancelsuspend on itself.

So the rule to remember is:

• Every use of suspend has the responsibility to have a plan for how some code, somewhere, will either cancelsuspend or resume the handle.
• Every use async has the responsibility to make sure that the resulting handle gets cancelasync or await applied to it.

Most user code will use async, await, and cancelasync, however there is occasionally the necessity to use suspend and cancelsuspend.