* numbers.h: Removed old comment about using SCM_CAR to access

* numbers.h:  Removed old comment about using SCM_CAR to access
non-pair cells.

(SCM_MOST_POSITIVE_FIXNUM, SCM_MOST_NEGATIVE_FIXNUM):  Make sure
the return value is signed.  Thanks to Brian Crowder for the bug
report.

(SCM_SRS):  Avoid unnecessary casting and don't unpack input
values.  With this patch, SCM_SRS can be safely used for other
types than scm_t_signed_bits.  However, it should still better be
an internal macro and thus be renamed to SCM_I_SRS.

(SCM_MAKINUM, SCM_INUM):  Use proper casting.