Caller breaking changes: Only use name to access children.

[fleming79]

Previously 'name' could be used to lookup the Caller, however this could result in name clashes.
Now with the concept of children, we restrict the usage of name for lookup to children preventing unexpected results.

Other changes:

• Caller.get() - now will always return a running caller, or raise an error if it isn't possible.
• async with Caller('new') as caller: the the new syntax to start a new caller with a context`
• Caller.get(name="MainThread") is now invalid. Use Caller.get("MainThread") instead.
• Changed from classmethod to method
  • Caller.to_thread
  • Caller.to_thread_advanced
  • Caller.as_completed
  • Caller.wait_for

Name changes:

• async_kernel.caller.Future -> async_kernel.pending.Pending
• Caller.current_future -> Caller.current_pending
• async_kernel.caller.FutureCancelledError -> async_kernel.pending.PendingCancelled