amazing this hasn't caused any problems before now

Contributed by: seliopou@gmail.com
    
This patch modifies darcs-all to have feature parity with push-all by
recognizing two new options.

    * -i, equivalent to --ignore-failure in push-all
    * -r <repo>, specifies the remote repository darcs commands will use

Some example commands:

Get the libraries from a repository of your choosing. This is useful
when working with a git mirror:

    $ ./darcs-all -r http://darcs.haskell.org get

Pull changes. Used to be:

    $ ./push-all --pull http://darcs.haskell.org

Is now:

    $ ./darcs-all -r http://darcs.haskell.org pull

Or to use the default remote of the ghc repository:

    $ ./darcs-all pull

This prints a combined hash of the ABIs exposed by the modules listed
on the command line.  It will be used by Cabal for generating a
package Id based on the actual ABI of a package.

We were making arrays with range (0, n-1) which is bad if n == 0 now
that we are using Word types.

We were keeping things as Int, and then converting them to Word16 at
the last minute, when really they ought to have been Word16 all along.

For inexpensive assertions

Instead of chr_$wchr, we now just get $wchr.  In general, when an
OccName is system-generated, we leave it out of the final external
name, preferring to use the name of the exported parent instead (which
is necessarily a user-written name).

Names should be no less deterministic, but should be shorter and more
readable.

It still lives in darcs, if anyone wants to revive it sometime.

Not a real fix: if this bites us again we'll have to rethink