postcard-dyn: reserialize into arbitrary Serializers

[max-heller]

Implements a postcard-dyn "reserializer" that deserializes postcard-serialized data and feeds it into an arbitrary Serializer in a streaming fashion (without needing to construct an intermediate representation in memory). I think this is (or could be) fully lossless. This means postcard -> serde_json::Value (or whatever generic representation postcard-dyn ends up providing), postcard -> json, and arbitrary other postcard -> X transformations can be performed by the same generic implementation.

A few implementation limitations:

• Serialize methods require &'static strs for structs/variants/field names. To provide these, the current implementation allocates and leaks one copy of each unique string that must be used as &'static. I can't find a way around this that isn't lossy and this seems inevitable in order to provide a Serializeable postcard_dyn::Value.
• This is implemented as a Serializeable struct that does the deserializing, which is the most straightforward way I found to implement the transformation. However, this means methods only receive &self, which necessitates use of RefCell for getting exclusive access to the Flavor.

@jamesmunns wanted to get your thoughts before I put more work into it. I think it's promising, but the first limitation is... rather limiting.

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