`hir` should know nothing about URLs, markdown and html. It should
only be able to:
* resolve stringy path from documentation
* generate canonical stringy path for a def
In contrast, link rewriting should not care about semantics of paths
and names resolution, and should be concern only with text mangling
bits.
Conjecture: it's impossible to use hir::Path *correctly* from an IDE.
I am not entirely sure about this, and we might need to add it back at
some point, but I have to arguments that convince me that we probably
won't:
* `hir::Path` has to know about hygiene, which an IDE can't set up
properly.
* `hir::Path` lacks identity, but you actually have to know identity
to resolve it correctly
