* eval.c, eval.h, evalext.c, evalext.h (scm_sym_setter,

	* eval.c, eval.h, evalext.c, evalext.h (scm_sym_setter,
	scm_m_generalized_set_x, scm_init_evalext): Move the declaration
	and definition of the memoizer for the generalized set! macro from
	evalext.[ch] to eval.[ch].  Use the SCM_SYNTAX snarfer macro to
	define the macro object.

	* eval.c, eval.h (s_set_x, scm_s_set_x, scm_m_set_x,
	scm_m_generalized_set_x): Since now scm_s_set_x is only used in
	eval.c, it is made static and renamed to s_set_x.

	* evalext.c (scm_defined_p, scm_m_undefine): Prefer !SCM_<foo>
	over SCM_N<foo>.