The zeros count for logcount on negatives can be done with mpz_hamdist
by passing -1 for the second operand.  This saves initializing and
clearing a temporary mpz_t.

	* numbers.c (z_negative_one): New variable.
	(scm_init_numbers): Initialize it.
	(scm_logcount): Use it and mpz_hamdist to count zeros for negatives.

	* tests/numbers.test (logcount): More tests.

I haven't changed the positive case, except to put it first and to
share the scm_remember_upto, but a few tests of that won't go astray.