fix a number of assuptions that a long could hold an inum

fix a number of assuptions that a long could hold an inum

* libguile/bytevectors.c:
* libguile/goops.c:
* libguile/instructions.c:
* libguile/numbers.c:
* libguile/random.c:
* libguile/read.c:
* libguile/vm-i-scheme.c: Fix a number of assumptions that a long could
  hold an inum. This is not the case on platforms whose void* is larger
  than their long.

* libguile/numbers.c (scm_i_inum2big): New helper, only implemented for
  sizeof(void*) == sizeof(long); produces a compile error on other
  platforms. Basically gmp doesn't have a nice interface for converting
  between mpz values and intmax_t.