Tease apart orthogonal concerns in markdown link rewriting

Tease apart orthogonal concerns in markdown link rewriting

`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.