Add struct class-level options - #39

Adds support for struct class-level options. These can be set as parameters to the metaclass. For example:

import msgspec

class Point(msgspec.Struct, immutable=True):
    x: float
    y: float

The following options are defined:

• immutable: makes a struct object immutable, and adds a __hash__ implementation.
• asarray: whether to serialize the struct as an array instead of as a dict.

Todo:

• Support passing options to the StructMeta metaclass
• Implement the immutable option
• Implement encoding Struct instances as arrays if asarray is set
• Implement decoding Struct instances from arrays if asarray is set
• Docs