Derive Serialize and Deserialize for most objects

[sfackler]

Before this PR

We manually implemented Serialize and Deserialize for all types, even if their representations could be handled by the normal serde derive macros.

After this PR

==COMMIT_MSG==
Serialize and Deserialize implementations of objects, aliases, and enums now use serde's derive macros.
==COMMIT_MSG==

This both shrinks the amount of code we're generating and should make it a bit more readable.

Unions are still manually implemented since serde doesn't support Conjure's weird combination of external and adjacent tagging. At some point we should move that logic into our own #[derive(Serialize_conjure)] macro.

[changelog-app]

Generate changelog in changelog/@unreleased

What do the change types mean?

• feature: A new feature of the service.
• improvement: An incremental improvement in the functionality or operation of the service.
• fix: Remedies the incorrect behaviour of a component of the service in a backwards-compatible way.
• break: Has the potential to break consumers of this service's API, inclusive of both Palantir services
  and external consumers of the service's API (e.g. customer-written software or integrations).
• deprecation: Advertises the intention to remove service functionality without any change to the
  operation of the service itself.
• manualTask: Requires the possibility of manual intervention (running a script, eyeballing configuration,
  performing database surgery, ...) at the time of upgrade for it to succeed.
• migration: A fully automatic upgrade migration task with no engineer input required.

Note: only one type should be chosen.

How are new versions calculated?

• ❗The break and manual task changelog types will result in a major release!
• 🐛 The fix changelog type will result in a minor release in most cases, and a patch release version for patch branches. This behaviour is configurable in autorelease.
• ✨ All others will result in a minor version release.

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Serialize and Deserialize implementations of objects, aliases, and enums now use serde's derive macros.

Check the box to generate changelog(s)

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