deprecate the-last-stack

deprecate the-last-stack

* libguile/backtrace.h (scm_the_last_stack_fluid_var)
* libguile/backtrace.c (scm_init_backtrace): No more
  scm_the_last_stack_fluid_var. The replacement is to resolve
  `the-last-stack' in (ice-9 stack-catch).
  (scm_backtrace_with_highlights): Accordingly, instead of backtracing
  the last stack, backtrace the current stack.

* libguile/throw.h:
* libguile/throw.c:
* libguile/deprecated.h:
* libguile/deprecated.c (scm_internal_stack_catch): Deprecate this
  function.

* module/ice-9/save-stack.scm (the-last-stack): Move here from boot-9.

* module/ice-9/debug.scm:
* module/ice-9/debugger.scm: Use (ice-9 save-stack) for the-last-stack.

* module/ice-9/deprecated.scm (the-last-stack): Add deprecated shim.