Update serde to 1.0.42
Update serde to 1.0.42

I ran:

    cargo update -p serde
    cargo update -p serde_derive
    cargo update -p serde_json
    cargo update -p quote

These new versions produce better code for serializing adjacently tagged
newtype variants, deserializing adjacently tagged unit variants, and
error handling in general. The total size of serde-related symbols in
the xi-core release binary (as measured by the following command)
decreases by 12% on my machine.

    nm -S target/release/xi-core \
      | awk '/serde/{sum += strtonum("0x"$2)} END{print sum}'

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