Very simple approach: For each identifier, set the hash of the range
where it's defined as its 'id' and use it in the VSCode extension to
generate unique colors.
Thus, the generated colors are per-file. They are also quite fragile,
and I'm not entirely sure why. Looks like we need to make sure the
same ranges aren't overwritten by a later request?
this is a part of larghish hir refactoring which aims to
* replace per-source-root module trees with per crate trees
* switch from a monotyped DedId to type-specific ids
