Replace C++ (in)equality checks with proper SCM syntax

Replace C++ (in)equality checks with proper SCM syntax

This commit replaces the most straightforward situations
where two SCM objects are compared.  Here are the basic
replacements used:

x == y                  ------->            scm_is_eq (x,y)
x == SCM_BOOL_T         ------->            to_boolean (x)
x == SCM_BOOL_F         ------->            scm_is_false (x)
x != SCM_BOOL_T         ------->            scm_is_true (x)
x == SCM_UNDEFINED      ------->            SCM_UNBNDP (x)
x == SCM_EOL            ------->            scm_is_null (x)
(scm_exact_p (x) == SCM_BOOL_T)  ------->   ly_is_rational (x)
(scm_integer_p (x) == SCM_BOOL_T) ------->  scm_is_integer (x)
(scm_list_p (x) == SCM_BOOL_T)  ------->    ly_is_list (x)
(scm_port_p (x) == SCM_BOOL_T)  ------->    ly_is_port (x)
(scm_equal_p (x, y) == SCM_BOOL_T) -------> ly_is_equal (x,y)

Finally, I replaced (!cached) with (!SCM_UNPACK (cached)) in
lily-guile-macros.hh, as has been suggested once by David.

Please note that this commit does not handle some situations
I’m less comfortable with (namely those involving scm_c_memq,
scm_assq or scm_hashq_get_handle), and some scm_*_p checks
for which there isn’t a ly_is_* replacement yet (to wit,
scm_hash_table_p, scm_promise_p, and scm_variable_bound_p).
Those should be dealt with later (another patch is in the works).