* numbers.c (scm_mkbig): Add cast, and note that signed/unsigned

* numbers.c (scm_mkbig): Add cast, and note that signed/unsigned
comparison is okay here.
* numbers.c (scm_istr2int): Add cast; len is known to be positive.
* numbers.c (scm_bigcomp): Clarify logic, and avoid relying on the
(true, but confusing) fact that -1 == ((unsigned) 0 - 1).
* numbers.c (scm_adjbig): Make nsiz an scm_sizet, to avoid mixing
signed/unsigned.