Add library procedure boolean? defined in 6.3.1  Booleans.

library procedure:  (boolean? obj)

http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-2.html#%_toc_%_sec_6.3.1
Treat #f as a false value and everything else as a true value.

This patch fixes a regression introduced in 631e4c4c94e6ab89fe8059b184e265f8a1a479c6.

The R5RS standard defines "a true value" and "a false value" in 6.3.1  Booleans:

	Of all the standard Scheme values, only #f counts as false in
	conditional expressions. Except for #f, all standard Scheme values,
	including #t, pairs, the empty list, symbols, numbers, strings,
	vectors, and procedures, count as true.