Using a git submodule for Diplomat

[robertbastian]

This makes it easier to iterate on ICU4X and Diplomat in parallel. It also simplifies CI as it can use the local diplomat-tool instead of having to cargo install it.

[sffc]

Not opposed but we should discuss this because adding a git submodule increases the complexity for everyone hacking on ICU4X even if they don't interact with Diplomat/FFI.

[robertbastian]

Yes we should discuss.

[robertbastian]

I've achieved most of what I wanted to achieve here with rust-diplomat/diplomat#348 and #4197

[sffc]

Discussion:

• @sffc - The number of people this helps is smallish, and the number of people it annoys is large. If you need fast iteration on diplomat locally, you can patch it locally to use path dependencies.
• @robertbastian - The changes I made in upstream about dependencies make the git submodule mostly obsolete, because you can now patch it locally.

Conclusion: Not worth the submodule.

LGTM: @robertbastian @Manishearth @sffc