Fix `deserialize_any` and serde-transcode support

[sunny-g]

Two things are flawed with the current implementation:

• deserializing tuples just fails (since it delegates to deserialize_seq which expects a list. This is an easy problem to "solve" though it leaves much to be desired. Updated to just deserialize to a list in 51ccd12
• we cannot deserialize "anonymous" enums or structs at all (using serde-transcode):
  • the first thing it does is call deserialize_any on the provided Deserializer
  • its passes a Visitor impl that doesn't implement a visit_enum method
  • even if it did, it eventually will call a method on the associated (i.e. our) Serializer like serialize_unit_variant, which requires &'static str, which the Visitor (probably?) cannot provide