* eval.h (SCM_EVALIM2): New macro. Use it when a

* eval.h (SCM_EVALIM2): New macro.  Use it when a
immediate, literal constant should be evaluated.
* eval.c (scm_s_duplicate_formals): New error message string.
(scm_c_improper_memq): New function.
(scm_m_lambda): Check for duplicate arguments.
(scm_ceval, scm_deval): When executing a body: only cons a new
toplevel environment frame when it is different from the
existing one; use EVALCAR instead of SIDEVAL so that we can properly
check for empty combinations; use SCM_EVALIM2 for the same reason
in the non-toplevel loop.
(nontoplevel_cdrxnoap, nontoplevel_cdrxbegin, nontoplevel_begin):
New labels with the meaning of their non-"nontoplevel" partners,
but they are used when it is known that the body is not evaluated at
top-level.
(scm_apply, scm_dapply): use SCM_EVALIM2 to get proper error
reporting for empty combinations.