Outside of examples, fill in all edition attrs

[dtolnay]

This is extracted from #1256 in case these changes are easier to review separately.

This is effectively a fully exhaustive version of #1254. Due to #1251, specifying edition in each target is required in order for downstream workspaces importing rules_rust to get a usable edition for all of rules_rust's targets, instead of attempting to use the downstream workspace's default edition which won't necessarily be appropriate.

Note that under examples, I have added edition to the rust_register_toolchains in various WORKSPACE.bazel files rather than to each individual target. This reduces noise in the example projects. I have not done the same for the top-level WORKSPACE because doing so would mask trouble along the lines of #1254.

[UebelAndre]

Thanks!

[UebelAndre]

@dfreese do you have time to take another look at these PRs?

[UebelAndre]

In the absence of @dfreese, my understanding is that this generally meets their concerns and I think this and the related changes are ready to merge. @dtolnay can you rebase?

[dtolnay]

• Rebased to resolve conflict with process_wrapper: add support for terminating rustc after it emits rmeta. #1207, which added some new Rust targets requiring edition in util/process_wrapper and test/process_wrapper

[dtolnay]

• Rebased again to resolve conflict with feat: support extra deps in rust_doc_test #1286