Preliminary IceCat 68.2 package

[Mark H Weaver <mhw@netris.org>]

Hello fellow Guix,

I was finally able to successfully build a working IceCat-68.2 package.
The 'wip-icecat-68' branch on Savannah contains this preliminary work.

A few caveats:

* IceCat 68 has not yet been released upstream.  This is very much a
  work-in-progress, and does not currently meet the privacy-respecting
  standards of the IceCat project.  There are still several issues to be
  resolved.  In particular, it might be that some information is being
  sent to Google or Mozilla without your knowledge.

* Many earlier attempts to build it have failed due to non-deterministic
  failures in the build system, possibly due to a bug in the Cargo tool.
  I'm not sure how much luck was involved in my successful build.  Your
  mileage may vary.

I'd be curious to hear whether others are able to build it successfully,
and whether it works for them.  I'd be grateful if others would try
building it and report back on their experiences.

      Thanks,
        Mark

PS: Many thanks to Efraim Flashner <efraim@flashner.co.il> for
    contributing the package definition for 'rust-cbindgen', and to
    Amin Bandali <bandali@gnu.org> for producing the first preliminary
    working version of IceCat 68.