* eval.c, tags.h, print.c (SCM_IM_SLOT_REF, SCM_IM_SLOT_SET_X):

* eval.c, tags.h, print.c (SCM_IM_SLOT_REF, SCM_IM_SLOT_SET_X):
New isym operations.

* eval.c (SCM_IM_DISPATCH, SCM_IM_HASH_DISPATCH): Don't use
improper lists in the low-level representation, since that will
cause a begin to be prepended at macro expansion.

* eval.c (scm_cons_source): Version of cons which copies source
properties from an existing cell.
(scm_m_quote, scm_m_begin, scm_m_if, scm_m_set_x, scm_m_and,
scm_m_or, scm_m_case, scm_m_cond, scm_m_lambda, scm_m_letstar,
scm_m_do, scm_m_letrec, scm_m_let, scm_copy_tree): Use
scm_cons_source.