Double-cells - *** warning

[Mikael Djurfeldt <mdj at mdj dot nada dot kth dot se>]

I've now comitted a large change

* Michael Livshin's double-word patch

* Changes in the type system: representation of bignums, real and
  complex types

* Extensive rewriting and changes in numbers.c, unif.c and ramap.c.

This of course means that I may have introduced trouble.

On my machine, Guile spends a lot of time in GC after this change.  I
thought about maybe waiting to commit the change until I had a clear
idea of why.  I had a preliminary look at the code, which I think
looks good, so I'll now invite everybody in this party.  ;-)

There is at least one thing which should be fixed, however: there is
now only one scm_heap_size, but there should probably instead be one
per freelist so that their hep segments can grow independently.