[MERGE][0.19] change the pack interface to use tuples for names

[Andrew Bennetts]

James Westby wrote:
> On (08/08/07 13:04), Robert Collins wrote:
> > As my NEWS item describes, this allows packs to use tuples for names,
> > which makes it simple to use composite keys such as ('foo', 'bar') or
> > ('t', FILEID, REVISIONID).
> > 
> > This is wire compatible with packs in 0.18 - as packs are just a
> > container, writers and readers have to match anyway, but a writer using
> > a single-string name will generate the same bytes on the wire.
> 
> If it is wire compatible then it seems ok to let this one in, if no one
> has any objections.

None from me.

-Andrew.