allow compilation of #@2(1 2 3)

allow compilation of #@2(1 2 3)

* libguile/arrays.h:
* libguile/arrays.c (scm_from_contiguous_array): New public function,
  like scm_from_contiguous_typed_array but for arrays of generic Scheme
  values.

* libguile/vm-i-scheme.c (make-struct): Sync regs before making the
  struct, so if we get a GC the regs are on the heap.
  (make-array): New instruction, makes an generic (untyped) Scheme
  array.

* module/language/glil/compile-assembly.scm (dump-object): Correctly
  compile arrays.