simplify inline function infrastructure
* libguile/__scm.h (SCM_C_EXTERN_INLINE): Move this definition here,
from inline.h. We'd like to support inline function definitions in
more header files: not just inline.h.
(SCM_CAN_INLINE, SCM_INLINE, SCM_INLINE_IMPLEMENTATION): New
definitions.
* libguile/gc.h (SCM_GC_MALLOC, SCM_GC_MALLOC_POINTERLESS): Define these
wrappers, which redirect to the GC_MALLOC macros when building Guile,
and the scm_gc_malloc functions otherwise. A step towards getting
BDW-GC out of Guile's API.
* libguile/inline.h: Simplify, using SCM_INLINE,
SCM_INLINE_IMPLEMENTATION, and SCM_IMPLEMENT_INLINES. Also use the
new SCM_GC_MALLOC macros.