fix bitvectors after the array handle refactoring

fix bitvectors after the array handle refactoring

* libguile/uniform.h (scm_array_handle_uniform_element_bit_size): New
  public accessor.

* libguile/uniform.c (scm_array_handle_uniform_element_size): Better
  errors in non-byte-aligned arrays.
  (scm_uniform_vector_element_type, scm_uniform_vector_element_size)
  (scm_c_uniform_vector_ref, scm_c_uniform_vector_set_x):
  (scm_uniform_vector_to_list): Don't require byte-aligned access.

* libguile/bytevectors.c (scm_uniform_array_to_bytevector):
* libguile/arrays.c (scm_from_contiguous_typed_array):  Fix for
  uniform arrays whose element size is not a multiple of the byte size.