Add Extension Type support - #31

This adds support for both encoding and decoding msgpack extension types. This comes with 2 additional apis:

• A new msgspec.Ext type, representing the tuple of (code: int, data: bytes_like) that describes a msgpack extension.
• A new ext_hook parameter to msgspec.Decoder/msgspec.decode.

By default msgspec will deserialize all extension objects into msgspec.Ext objects. However, if a user specifies a ext_hook to msgspec.Decoder/msgspec.decode, this callback will be used instead to deserialize the extension type. Together with the existing default hook in msgspec.Encoder/msgspec.encode, this lets users fully add extra types to be serialized/deserialized.

For example, say we wanted to define a new extension type with code 42 that contains a buffer of data serialized via pickle. If we wanted to apply this to all non-builtin types serialized by msgspec, we could do this as follows:

import msgspec
import pickle
from typing import Any

def default(obj: Any) -> Any:
    """Serialize the object using pickle, and mark it as an extension type with code 42"""
    return msgspec.Ext(42, pickle.dumps(obj))

def ext_hook(code: int, data: memoryview) -> Any:
    if code == 42:
        return pickle.loads(data)
    raise TypeError(f"Unknown extension type with code {code}")

enc = msgspec.Encoder(default=default)
dec = msgspec.Decoder(ext_hook=ext_hook)

buf = enc.encode(range)  # functions normally aren't serializable with msgspec
out = dec.decode(buf)
assert out == range

Note that ext_hook has the signature ext_hook(code: int, data: memoryview) -> Any. Here data is a memoryview into larger message buffer. This lets us avoid an extra copy when forwarding the data buffer to ext_hook, but it means that if the returned object holds on to a reference to that buffer (instead of doing a copy itself) then the larger message will persist in memory until that object is freed. For most use cases of ext_hook I wouldn't expect this issue to come up, but it's worth noting. The performance improvement from avoiding the copy is worth it to add a tiny rare footgun IMO.

Fixes #26.

Still a few todos:

• Make Ext accept a memoryview for data, rather than just bytes or bytearray
• Add docs