Add support for default_factory on Struct types

[jcrist]

This PR does a few things:

• It adds a new msgspec.field function, which returns an opaque object used for configuring fields. So far this only exposes default and default_factory, both of which match the dataclasses kwargs of the same name.
• It adds support for init-time generated default values in msgspec.Struct types, through use of the new default_factory setting (fixes Support default factories for Struct types #259).

Example:

import msgspec
import uuid

class Example(msgspec.Struct):
    id: uuid.UUID = msgspec.field(default_factory=uuid.uuid4)

Breaking Change: mutable default values in structs are now handled differently

This also changes how mutable default values are handled. Previously msgspec's semantics were that a default value for a field would be deepcopied on __init__ if that field wasn't provided (the implementation would avoid a deepcopy for most common types, but semantically this was the same). This proved problematic in the face of custom types, led to some unnecessary slowdowns, and didn't match the behavior of other similar libraries like dataclasses or attrs. Since we now can support arbitrarily complex default values through the default_factory function, we drop the old deepcopy behavior.

A default value on a Struct now has the following rules:

• Common mutable containers (list, dict, set, bytearray) will error if used as the default value if they aren't empty. The error points users to use a default_factory instead.
• Empty common mutable containers ([], {}, set(), bytearray()) are accepted as default values. These are treated as syntactic sugar, and are automatically converted to the equivalent default_factory notation ([] -> field(default_factory=list)).
• Mutable struct types will error if used as default values. The error points users to use a default_factory instead.
• Frozen struct types are accepted as a default value, and are used directly without deepcopying.
• All other values are used directly without deepcopying

Since most mutable default values are empty collections, I don't expect this breaking change to affect most users. To reiterate, the following type definition is still valid:

class Example(Struct):
    x: list[int] = []
    y: set[int] = set()
    z: dict[str, int] = {}

while this one will error, and should use a default_factory instead:

class Bad(Struct):
    x: list[int] = [1, 2, 3]

# should be
class Good(Struct):
    x: list[int] = field(default_factory=lambda: [1, 2, 3])

Todo:

• Tests
• Docs
• Ensure all examples are updated to the newer syntax

[jcrist]

A quick benchmark of __init__ costs after this change, comparing msgspec to a few other dataclass-like libraries: https://gist.github.com/jcrist/9bfe44f60533225d5f8383791f2fe734. In short - msgspec is fast. Allocating a new struct even in the presence of default values now is as fast as allocating a tuple.