Add interface to disable automatic finalization

* libguile/finalizers.h:
* libguile/finalizers.c (run_finalizers_async_thunk): Call the new
  scm_run_finalizers helper.
  (scm_set_automatic_finalization_enabled, scm_run_finalizers): New
  functions.
  (scm_init_finalizers): Only set a finalizer notifier if automatic
  finalization is enabled.

* doc/ref/libguile-smobs.texi (Garbage Collecting Smobs): Add discussion
  of concurrency.

* doc/ref/api-smobs.texi (Smobs): Document new functions.
Fix foreign slot initialization and access

* libguile/goops.c (scm_sys_initialize_object): Refactor initialization
  so that we don't ref uninitialized slots before initializing them.
  This allows foreign slots, whose initial value is 0, to be initialized
  via #:init-form.

* module/oop/goops.scm (@slot-ref, @slot-set!): Remove definitions.
  Change callers to use struct-ref and struct-set!.  slot-ref and
  slot-set! were only marginally more efficient and were much more
  dangerous.  This change allows the standard accessors to work on
  foreign slots; that was not the case before, as the 'u' fields of the
  struct were read as if they were 'p' slots.
* module/language/tree-il/compile-glil.scm (lambda): Remove support for
  compiling @slot-ref/@slot-set!.  These were private to GOOPS.

* test-suite/tests/goops.test ("active-slot"): Update to not expect a
  ref before initialization.
  ("foreign slots"): Add tests.