...well, most of them anyhow...

I got tired of submitting these piecemeal, and it's probably easier to 
review them in one go.

There should be no difference in functionality, barring an extra call to 
set_and_canonicalize_value_range (instead of set_value_range) due to the 
way I've organized multiplication and lshifts for maximal sharing.  This 
also gets rid of some repetitive stuff.

I've also added a value_range::dump like wide_int::dump.  It makes 
debugging a lot easier.

My next patch will move all the wide_int_range_* stuff into 
wide-int-range.[hc].

I'm really liking how this is turning out, BTW: a *lot* cleaner, less 
code duplication, and shareable to boot :).

OK pending one more round of tests?

Aldy