optimize access to arrays of rank 1 or 2

optimize access to arrays of rank 1 or 2

* libguile/array-handle.c (scm_array_handle_pos_1):
  (scm_array_handle_pos_2): New functions.

* libguile/generalized-arrays.c (scm_c_array_ref_1, scm_c_array_ref_2):
  (scm_c_array_set_1_x, scm_c_array_set_2_x): New functions.
  (scm_i_array_ref, scm_i_array_set_x): New subr bindings for array-ref
  and array-set! that avoid consing for arrays of rank 1 or 2.

* test-suite/tests/arrays.test ("array-set!"): Fix expected exception
  for wrong number of indices.