add create_priority_memory_object_stream

[graingert]

Changes

Previously this was required for a Textual port to AnyIO. This is just a sketch to see how extensive the changes would be.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[graingert]

not sure if max_buffer_size should be allowed to be set to 0 or 1 for create_priority_memory_object_stream, it would behave the same as create_memory_object_stream, but be slower for no purpose.

we could set the default to be math.inf and require it to be more than 2