Refactor Cocoa stuff into a 'receiver'

The idea here is that there are other places that might want to know
what's playing, besides MPNowPlayingInfoCenter. For example, to expose
the now playing info to Übersicht efficiently, it needs to be available
from a web browser, ideally using WebSockets. So there could be a
receiver that runs a small WebSockets server and sends out now playing
info to anyone who connects.

Additionally, I hope to write receivers for MPRIS and for the System
Media Transport Controls on Windows, making mpd-now-playable equally
useful across all platforms.

None of this is implemented yet, of course, but I hope to get the
WebSockets receiver done pretty soon!

I'm going to keep the default behaviour unchanged. Unless you
explicitly configure different receivers in config.toml,
mpd-now-playable will just behave as an MPNowPlayingInfoCenter
integration as it's always done.

src/mpd_now_playable/receivers/cocoa/persistent_id.py

@@ -0,0 +1,50 @@
+from hashlib import blake2b
+from pathlib import Path
+from typing import Final
+from uuid import UUID
+
+from ...song import Song
+
+# The maximum size for a BLAKE2b "person" value is sixteen bytes, so we need to be concise.
+HASH_PERSON_PREFIX: Final = b"mnp.mac."
+TRACKID_HASH_PERSON: Final = HASH_PERSON_PREFIX + b"mb_tid"
+RELEASETRACKID_HASH_PERSON: Final = HASH_PERSON_PREFIX + b"mb_rtid"
+FILE_HASH_PERSON: Final = HASH_PERSON_PREFIX + b"f"
+
+PERSISTENT_ID_BITS: Final = 64
+PERSISTENT_ID_BYTES: Final = PERSISTENT_ID_BITS // 8
+
+
+def digest_trackid(trackid: UUID) -> bytes:
+	return blake2b(
+		trackid.bytes, digest_size=PERSISTENT_ID_BYTES, person=TRACKID_HASH_PERSON
+	).digest()
+
+
+def digest_releasetrackid(trackid: UUID) -> bytes:
+	return blake2b(
+		trackid.bytes,
+		digest_size=PERSISTENT_ID_BYTES,
+		person=RELEASETRACKID_HASH_PERSON,
+	).digest()
+
+
+def digest_file_uri(file: Path) -> bytes:
+	return blake2b(
+		bytes(file), digest_size=PERSISTENT_ID_BYTES, person=FILE_HASH_PERSON
+	).digest()
+
+
+# The MPMediaItemPropertyPersistentID is only 64 bits, while a UUID is 128
+# bits and not all tracks will even have their MusicBrainz track ID included.
+# To work around this, we compute a BLAKE2 hash from the UUID, or failing
+# that from the file URI. BLAKE2 can be customised to different digest sizes,
+# making it perfect for this problem.
+def song_to_persistent_id(song: Song) -> int:
+	if song.musicbrainz_trackid:
+		hashed_id = digest_trackid(song.musicbrainz_trackid)
+	elif song.musicbrainz_releasetrackid:
+		hashed_id = digest_releasetrackid(song.musicbrainz_releasetrackid)
+	else:
+		hashed_id = digest_file_uri(song.file)
+	return int.from_bytes(hashed_id)