The indirection uses a "local-redirect.js" script to rewrite
the document links to local links within the browser. This mechanism
is a step towards distributing compiled packages that include
already-built documentation, where paths to other documentation
can be different than in the build environment.
If the links are not rewritten, they are queries to
"pkg-docs.racket-lang.org", with the idea tha such a server will exist
for reading all package documentation online. Also, a package's
documentation that refer to documentation for uninstalled packages,
in which case the corresponding links will not get rewritten and
will continue to point to the server.
Rendering the "local-redirect.js" script spends a lot of time just
converting among different path formats. Various library changes in
this commit are aimed at speed up those conversions, but the big
improvement came from a `path->url-string' that shortcuts conversion
os simple Unix paths.
