postcard-schema: how to handle schemas conditional on is_human_readable()

[max-heller]

Some Serialize/Deserialize implementations (see e.g. uuid::Uuid) use different serde data model representations of the type depending on whether the underlying Serializer/Deserializer is human readable. For the most part, postcard should be able to think only in terms of the non-human-readable representation, but the distinction may matter for postcard-dyn in order to properly deserialize from human readable formats or serialize to human readable formats (e.g., json).

This could be captured in the schema as something like an IfHumanReadable { then: Type, else: Type } schema variant, though that feels somewhat clunky.

[max-heller]

Unfortunately postcard-dyn is probably out of luck here as there's no guarantee that serialized data can be transformed between its human-readable and compact forms, let alone in a universal way.