unbound fluids

unbound fluids

* libguile/fluids.c (scm_make_undefined_fluid, scm_fluid_unset_x)
  (scm_fluid_bound_p): New functions.

  (fluid_ref): New function; like scm_fluid_ref, but will not throw an
  error for unbound fluids.
  (scm_fluid_ref, swap_fluid): Use `fluid_ref'.

* libguile/fluids.h (scm_make_undefined_fluid, scm_fluid_unset_x)
  (scm_fluid_bound_p): New prototypes.

* libguile/vm-i-system.c (fluid_ref): If fluid is unbound, jump to
  `vm_error_unbound_fluid'.
* libguile/vm-engine.c (VM_NAME)[vm_error_unbound_fluid]: New error
  message.

* test-suite/tests/fluids.test ("unbound fluids")["fluid-ref of unbound
  fluid", "fluid-bound? of bound fluid", "fluid-bound? of unbound
  fluid", "unbound fluids can be set", "bound fluids can be unset"]: New
  tests.