add support for de/serializing objects with json and msgpack

[anthrotype]

This adds methods for serializing / deserializing to/from JSON, MessagePack and Pickle to all the ufoLib2.objects (Font, Glyph, etc.).
It's mostly syntactic sugar around the ufoLib2.converters' structure/unstructure which provides convenience one-line methods for serialization to popular formats.

E.g.

font = Font.json_load("path/to/Font1.json")
font2 = Font.json_loads(font.json_dumps())
font2.json_dump("path/to/Font2.json")

or

font = Font.msgpack_load("path/to/Font1.msgpack")
font2 = Font.msgpack_loads(font.msgpack_dumps())
font.msgpack_dump("path/to/Font2.msgpack")

Json serialization uses orjson on CPython as it's faster than built-in json. Optional dependencies (e.g. msgpack) are handled via extras.

It's just a proof-of-concept to solicit feedback, I simply drop it here in case it may be of use in our current effort to break up fontmake into parallel processes in order to speed it up.

[anthrotype]

Added some tests, I think this is now ready to review

[anthrotype]

One thing this PR adds which I didn't mention in the description is the ability to pickle/unpickle ufoLib2 objects that are "lazy", i.e. they contain references to filesystem objects (via UFOReader or glifLib.GlyphSet) which aren't pickleable as such (because they hold a threading.RLock and that apparently is by design in pyfilesystem2).
So to make lazy objects pickleable I do like we already do with deepcopy, that is I run unlazify() on them before handling them to pickle.

[anthrotype]

I have one question for typing enthusiast like @madig

currently, since I add the json_loads/json_dumps/json_load/json_dump etc. methods dynamically to the ufoLib2 objects using the @serde class decorator, static typecheckers like mypy can't see them hance the deluge of # type: ignore whenever I use them.
I don't see how I can make mypy aware of these. Could we write typing stubs maybe? I have no idea.

So it's a bit inconvenient if one relies on typechecking to having to opt-out every time one uses these new de/serialization methods.

One alternative is to not use the class/instance methods added by the @serde decorators, but directly call the loads/dumps functions defined respectively in ufoLib2.serde.json and ufoLib2.serde.msgpack, which have type hints and don't make mypy angry.

E.g. instead of doing

from ufoLib2 import Font

font = Font.json_load("MyFont.json")  # type: ignore
font.json_dump("MyFont2.json")  # type: ignore

one could do instead:

from pathlib import Path
import ufoLib2.serde.json
from ufoLib2 import Font

font = ufoLib2.serde.json.loads(Path("MyFont.json").read_bytes(), Font)

Path("MyFont2.json").write_bytes(ufoLib2.serde.json.dumps(font, indent=2))

It's a bit more verbose and indirect but makes mypy happy.

[madig]

LGTM as far as I can see 👀