Support omitting Struct attributes from encoding/decoding #199

Since Struct types force users to declare all Struct attributes, there's currently no way to hold extra state on a Struct without having it also be encoded/decoded.

Two ways to handle this:

• Add omit=True to Meta annotations. This could also be packaged up as a parametrized type, so users could write

class Point(Struct):
    x: int
    y: int
    _private: Omit[some_type]

• Excluded attributes with a leading underscore by default? I'm not sure if we should support this one, it may be confusing behavior. As a reference point, golang's json doesn't encode/decode private struct attributes by default.