[postcard-derive]: `MaxSize` codegen is too simple

[jamesmunns]

HaoboGu/rmk#691 is an issue that shows that when deriving MaxSize on an enum, if the enum has lots of variants, we generate a LOT of code and rustc runs out of stack.

We should improve this, even if the RMK issue is fixed by avoiding the derive.

[jamesmunns]

https://github.com/HaoboGu/rmk/blob/e7577ea3e28d7f54cd17f291af3f446bb71b8464/rmk-types/src/keycode.rs#L11-L863 for a ~300 item enum