Add dec_hook to Decoder/decode

[jcrist]

Adds a new dec_hook callback supporting type conversions between
builtin msgpack types and arbitrary python types. This hook should have
the following signature:

def dec_hook(obj: Any, type: Type) -> Any:
    ...

This receives an object composed of builtin msgpack types (e.g. dict,
list, int, ...) and the expected decode type (as described by the
decoder's type parameter). It should attempt to convert the value to the
provided type, or raise a type error appropriately. When combined with
enc_hook on the encoder, this lets users serialize arbitrary custom
objects without resorting to extension types.

For example, to serialize/deserialize a NamedTuple you might write
the following:

import msgspec
from typing import NamedTuple

class Point(NamedTuple):
    x: int
    y: int

def enc_hook(obj):
    if isinstance(obj, tuple):
        # convert the named tuple to a supported type
        # in this case it's a tuple like `(1, 2)`
        return tuple(obj)
    raise TypeError(f"Type {type(obj).__name__} is not supported")

def dec_hook(obj, type):
    if issubclass(type, tuple):
        # convert the builtin type (e.g. `[1, 2]`) to
        # the custom type (`Point(x=1, y=2)`)
        return type(*obj)
    raise TypeError(f"Type {type.__name__} is not supported")

enc = msgspec.Encoder(enc_hook=enc_hook)
dec = msgspec.Decoder(Point, dec_hook=dec_hook)

msg = enc.encode(Point(1, 2))
x = dec.decode(msg)
print(x)  # Point(x=1, y=2)

There's still a few TODO items, but this is mostly functional already

• Tests
  - [ ] Docs
• Better support GenericAlias objects for custom types

[jcrist]

TODO: switch the order of parameters in dec_hook. Since the user will most likely be dispatching on the type, it'd be nice if this parameter was first to mirror the convention in other dispatch decorators (e.g. functools.singledispatch).

[jcrist]

Putting off writing docs til a later PR. This is good for now. Merging.