fixnum != fixnum [was ping numbers.pika.hackerlab]

The latest patch in my tree "basic fixnum support" is bad and evil -
don't merge it yet... here's why:

Tom Lord <lord-1tv/[email protected]> writes:

>     --------------------------------
>     |                              |
>     |  scm/libscm-numbers-reps     |      scm_reps_add normally adds two
>     |                              |      numbers but can also instead 
>     | implements math routines     |      return scm_err_numeric_raise_needed
>     | and conversion <-> C types   |
>     |                              |      in that case, scm_add converts
>     | can add new numeric reps     |      the arguments to the same 
>     | using the vtable facility    |      representation and retries
>     |                              |      the add
>     --------------------------------
>     |                              |
>     |  scm/reps                    |
>     |                              |      fixnum and double representations
>     |  provides optimized          |      are coded here so that they
>     |  representations for         |      can be heavily optimized
>     |  fixnums and doubles         |
>     |                              |      other new number types just
>     |  (also provides GC,          |      use the `vtable' facility
>     |   vtable objects, etc.)      |
>     --------------------------------

Something that eluded me until just now is that earlier when we
discussed numbers implementations on IRC, we referred to small
integers embedded in t_scm_words as fixnums and integers that needed
to be heap allocated as bignums - I must have missed the phase when we
started referring to both as fixnums.

So, just to clarify things, the libscm-numbers-reps fixnum code is
responsible for dispatch to reps fixnum when it can fit the result
into a t_scm_word, but otherwise must create the bignum data.

-jivera