[Concurrency] Alternative Task API.

[DougGregor]

The Task type has oscillated somewhat from being purely a namespace,
to having instances that are used (albeit rarely), back to purely
being a namespace that isn't used for all that many names. Many of the
names that used to be on Task have already been moved out, e.g., for
creating new detached tasks, creating new task groups, adding
cancellation handlers, etc.

Collapse Task.Handle<Success, Failure> into Task<Success, Failure>.
Task.Handle is the type that is most frequently referenced in the
concurrency library, so giving it the short name Task is most
appropriate. Replace the top-level async/detach functions with a
Task initializer and Task.detached, respectively.

The Task type can still act as a namespace for static operations
such as, e.g., Task.isCancelled. Do this with an extension of the
form:

extension Task where Success == Never, Failure == Never { ... }

We've been accruing a number of compatibility shims. Move them all
into their own source file, deprecate them, and make them
always-emit-into-client so they don't have any ABI impact.

This also covers other changes that came up in the review:

• Switch get() to value as an async property
• Switch getResult() to result as an async property
• Switch order/names of withTaskCancellationHandler parameters
• Update names of TaskPriority members, and make it a RawRepresentable struct

[DougGregor]

@swift-ci please test

[ktoso]

Looking good! Thanks Doug, pretty happy with the final names of things.

Minor change to be done in priorities still, you can do this here or later.

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please build toolchain