Add fromFutureCancelable and friends

[armanbilge]

This is a follow-up to:

• Change async_ to be uncancelable #3205

I was working on http4s-dom and encountered roughly this:

F.fromPromise(F.delay(Fetch.fetch(...))).onCancel(F.delay(abortController.abort()))

The problem is that fromPromise is implemented via async_ which will be uncancelable under the new semantics. Without a fromPromiseCancelable variant I'd have to re-implement the conversion logic or run it on another fiber, so here we are.

Along these lines I also added fromFutureCancelable. I did not add a variant for CompletableFuture because its API natively supports cancelation (go Java! ). As a plus side, these methods offer an escape hatch for anyone who is bitten by the new semantics.

The method names and signatures feel a bit unergonomic so definitely open to bikeshed. I didn't try too hard to share implementations since it would also be messy.

[djspiewak]

The nesting on this pretty much prevents anything with better ergonomics. I think this is probably the best we can do, unfortunately.

[He-Pin]

lol:) refs: #160 too