* fluids.c (scm_c_with_fluids): Use frames instead of adding to

* fluids.c (scm_c_with_fluids): Use frames instead of adding to
the wind chain explicitely.  Use scm_c_with_fluid for the common
case of only one fluid.
(scm_with_fluid): New.
(scm_c_with_fluid): Use frames instead of scm_c_with_fluids.

* fluids.h, fluids.c (scm_frame_fluid): New.
(scm_with_fluid): New.
(scm_i_swap_fluids, scm_i_swap_fluids_reverse): Removed.