Support keyword-only parameters in structs - #242

This adds support for keyword-only parameters in Structs, via a kw_only configuration option. This option has the same semantics and behavior as the kw_only parameter for dataclasses. If true, all parameters specified on the respective class (not base classes) are keyword-only.

In [1]: import msgspec

In [2]: class Point(msgspec.Struct, kw_only=True):
   ...:     x: int
   ...:     y: int
   ...:     z: int = 0
   ...: 

In [3]: Point.__signature__
Out[3]: <Signature (*, x: int, y: int, z: int = 0)>

This commit also changes the field-ordering algorithm used by msgspec to match the behavior of dataclasses. This is a Breaking Change, but helps maintain compatibility with the larger Python ecosystem (it also makes pyright type check fields with the proper order in the presence of subclasses).

The main way this breaking change may affect users is by causing struct definitions with required parameters defined after optional ones to fail at import time.

For example, the following definition is now invalid and will error at import time.

class MyStruct(Struct):
    x: int = 1  # an optional parameter
    y: int  # a required parameter

To be clear, dataclasses & attrs have this same restriction - only msgspec lacked this restriction before, but now has it for compatibility with dev tools like pyright

To resolve this issue, you can either:

• reorder the fields so x is after y
• add a default value to y
• Specify kw_only=True in the struct definition. Keyword-only fields may mix required and optional fields in any order, but impose limitations on the caller.

class MyStruct(Struct, kw_only=True):
    x: int = 1
    y: int

x = MyStruct(x=10, y=20)  # this works

It is unfortunate that a breaking change was needed here, but the compatibility with dataclasses (and pyright) feels worth it. Better break things now than later.