* libguile/eval.c (scm_m_let, scm_m_letstar, scm_m_letrec,

	* libguile/eval.c (scm_m_let, scm_m_letstar, scm_m_letrec,
	scm_m_expand_body, check_bindings): Extracted syntax checking of
	bindings to new static function check_bindings.

	(scm_m_let, memoize_named_let): Extracted handling of named let to
	new static function memoize_named_let.

	(transform_bindings, scm_m_let, scm_m_letstar, scm_m_letrec): Use
	ASSERT_SYNTAX to signal syntax errors.  Be more specific about the
	kind of error that was detected.  Avoid use of SCM_CDRLOC.  Avoid
	unnecessary consing when creating the memoized code.

	* test-suite/lib.scm (exception:bad-variable): New.

	* test-suite/tests/syntax.test (exception:bad-binding,
	exception:duplicate-binding): New.

	(exception:duplicate-bindings): Removed.

	Adapted tests for 'let', 'let*' and 'letrec' to the new way of
	error reporting.