[Concurrency] Task namespace, priority and always throwing Task.Handle.get

[ktoso]

rdar://70141994

This is a follow up after discussions on these pieces of the API;

Each commit should be self explanatory what the change is; the primary one being:

• Task after all is only a namespace; we never get "a task" object in user land; all interaction with "my task" is via async context sensitive static functions
• added a placeholder for priority -- not filled in as we're not entirely sure about its shape yet
• Task.Handle.Failure was removed, as there is no meaningful way to constrain it because we have to allow CancellationError to be thrown, even if a referred to task is not throwing.

Follow up to: #34391

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please test

[ktoso]

Merging this to unblock new PRs -- it's a simple follow up to the previous PR and addresses changes requested there and on offline discussions.