Make some errors serializable

[sagudev]

Description
In servo we need errors to be serializable.

Reviewable per commits (each commit builds by itself)

Testing
It compiles.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[ErichDonGubler]

I don't see any blocking issues with this. I want to tag in @cwfitzgerald for thoughts on supporting this as a temperature check, but I don't see any issues with the implementation beyond what I've submitted comments for.

[cwfitzgerald]

No problems with this landing, the only thing I'm unsure about is that these serializes have no in-codebase justification, so could be broken at some point. I don't know what to do about that problem, just noting it.

[sagudev]

No problems with this landing, the only thing I'm unsure about is that these serializes have no in-codebase justification, so could be broken at some point. I don't know what to do about that problem, just noting it.

We could add test (or const asserts) that some types implements (de)serialize (like it's done for Sync/Send), but I think that would make things complicated. (de)serialize can be trivially added again if needed by consumers (mostly browsers).

[cwfitzgerald]

Lezdoit