Fix deserialization of Smile maps with UUID keys

[sfackler]

Before this PR

The Smile serde Deserializer used for map keys incorrectly reported itself as not "human readable", which causes UUID deserialization logic to expect raw bytes instead of the string encoding. While maps keyed by UUIDs would serialize properly, they would not deserialize, failing with

Error(Custom("invalid value: string \"820eea12-894b-4e6e-96a0-668b79b9edb4\", expected a base64 string"))

After this PR

==COMMIT_MSG==
==COMMIT_MSG==

We now override is_human_readable to true in the key deserializer instead of delegating to the underlying Deserializer.

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Description

Fix deserialization of Smile maps with UUID keys

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