Update serde to 1.0.39 and serde_json to 1.0.15
Update serde to 1.0.39 and serde_json to 1.0.15

I ran:

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

These new versions produce better code for adjacently tagged enums.
The total size of serde-related symbols in the debug binary (as
measured by the following command) decreases by 22% on my machine.

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

google/xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,google/xi-editor