Support __post_init__ for Struct types

[jcrist]

This adds support for an optional __post_init__ hook method on msgspec.Struct types. If defined, this method is called at the end of the generated __init__ method. It can be used to customize the builtin __init__ as needed, as well as provides a spot to do some additional validation.

As with dec_hook errors, any ValueError or TypeError raised in __post_init__ will be wrapped in a ValidationError when decoding. In the long run I plan to add support for more granular (and obviously named) custom validator support, but for now doing some validation in __post_init__ is a fine and simple solution. There are other use cases for __post_init__ besides validation, so we'd want to add this ability either way.

Fixes #442.

[jcrist]

A quick example:

>>> import msgspec

>>> class Interval(msgspec.Struct):
...     low: float
...     high: float
...
...     def __post_init__(self):
...         if self.low > self.high:
...             raise ValueError("`low` may not be greater than `high`")

>>> Interval(1, 2)  # valid interval
Interval(low=1, high=2)

>>> Interval(2, 1)  # invalid interval
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __post_init__
ValueError: `low` may not be greater than `high`

>>> msgspec.json.decode(b'{"low": 2, "high": 1}', type=Interval)  # invalid interval from JSON
Traceback (most recent call last):
  File "<stdin>", line 6, in __post_init__
ValueError: `low` may not be greater than `high`

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
msgspec.ValidationError: `low` may not be greater than `high`

[akotlar]

Amazing, thank you!