And allow `non_local_definitions`.

Co-authored-by: Joshua Liebow-Feeser <joshlf@users.noreply.github.com>

Previously, we had avoided doing this because there wasn't any way to
guarantee that we didn't make mistakes and render the wrong version for
a particular named toolchain version.

In this commit, we take the pragmatic approach of renaming the named
toolchain versions - in particular, we add a suffix which includes the
toolchain version. This is fairly error-resistant for two reasons:
- At the definition site (in `Cargo.toml`), the name appears on the same
  line as the version, so mistakes should be obvious
- At the use site (in various `.rs` files), the name appears within a
  few lines of any `#[cfg_attr(doc_cfg, doc(cfg(...)))]` attribute, so
  mistakes should similarly be obvious

Release 0.8.7.

Closes #894

gherrit-pr-id: I74b8e9be0e6cb6e02552d2e1f985b490a7d37afe