* libguile/array-handle.c (initialize_vector_handle): Add mutable_p
  argument.  Unless the vector handle is mutable, null out its
  writable_elements member.
  (scm_array_get_handle): Adapt to determine mutability of the various
  arrays.
  (scm_array_handle_elements, scm_array_handle_writable_elements):
  Reverse the sense: instead of implementing read-only in terms of
  read-write, go the other way around, adding an assertion in the
  read-write case that the array handle is mutable.
* libguile/array-map.c (racp): Assert that the destination is mutable.
* libguile/bitvectors.c (SCM_F_BITVECTOR_IMMUTABLE, IS_BITVECTOR):
  (IS_MUTABLE_BITVECTOR): Add a flag to indicate immutability.
  (scm_i_bitvector_bits): Fix indentation.
  (scm_i_is_mutable_bitvector): New helper.
  (scm_array_handle_bit_elements)
  ((scm_array_handle_bit_writable_elements): Build writable_elements in
  terms of elements.
  (scm_bitvector_elements, scm_bitvector_writable_elements): Likewise.
  (scm_c_bitvector_set_x): Require a mutable bitvector for the
  fast-path.
  (scm_bitvector_to_list, scm_bit_count): Use read-only elements()
  function.
* libguile/bitvectors.h (scm_i_is_mutable_bitvector): New decl.
* libguile/bytevectors.c (INTEGER_ACCESSOR_PROLOGUE):
  (INTEGER_GETTER_PROLOGUE, INTEGER_SETTER_PROLOGUE):
  (INTEGER_REF, INTEGER_NATIVE_REF, INTEGER_SET, INTEGER_NATIVE_SET):
  (GENERIC_INTEGER_ACCESSOR_PROLOGUE):
  (GENERIC_INTEGER_GETTER_PROLOGUE, GENERIC_INTEGER_SETTER_PROLOGUE):
  (LARGE_INTEGER_NATIVE_REF, LARGE_INTEGER_NATIVE_SET):
  (IEEE754_GETTER_PROLOGUE, IEEE754_SETTER_PROLOGUE):
  (IEEE754_REF, IEEE754_NATIVE_REF, IEEE754_SET, IEEE754_NATIVE_SET):
  Setters require a mutable bytevector.
  (SCM_BYTEVECTOR_SET_FLAG): New helper.
  (SCM_BYTEVECTOR_SET_CONTIGUOUS_P, SCM_BYTEVECTOR_SET_ELEMENT_TYPE):
  Remove helpers.
  (SCM_VALIDATE_MUTABLE_BYTEVECTOR): New helper.
  (make_bytevector, make_bytevector_from_buffer): Use
  SCM_SET_BYTEVECTOR_FLAGS.
  (scm_c_bytevector_set_x, scm_bytevector_fill_x)
  (scm_bytevector_copy_x): Require a mutable bytevector.
* libguile/bytevectors.h (SCM_F_BYTEVECTOR_CONTIGUOUS)
  (SCM_F_BYTEVECTOR_IMMUTABLE, SCM_MUTABLE_BYTEVECTOR_P): New
  definitions.
* libguile/bytevectors.h (SCM_BYTEVECTOR_CONTIGUOUS_P): Just access one
  bit.
* libguile/srfi-4.c (DEFINE_SRFI_4_C_FUNCS): Implement
  writable_elements() in terms of elements().
* libguile/strings.c (scm_i_string_is_mutable): New helper.
* libguile/uniform.c (scm_array_handle_uniform_elements):
  (scm_array_handle_uniform_writable_elements): Implement
  writable_elements in terms of elements.
* libguile/vectors.c (SCM_VALIDATE_MUTABLE_VECTOR): New helper.
  (scm_vector_elements, scm_vector_writable_elements): Implement
  writable_elements in terms of elements.
  (scm_c_vector_set_x): Require a mutable vector.
* libguile/vectors.h (SCM_F_VECTOR_IMMUTABLE, SCM_I_IS_MUTABLE_VECTOR):
  New definitions.
* libguile/vm-engine.c (VM_VALIDATE_MUTABLE_BYTEVECTOR):
  (VM_VALIDATE_MUTABLE_VECTOR, vector-set!, vector-set!/immediate)
  (BV_BOUNDED_SET, BV_SET): Require mutable bytevector/vector.
* libguile/vm.c (vm_error_not_a_mutable_bytevector):
  (vm_error_not_a_mutable_vector): New definitions.
* module/system/vm/assembler.scm (link-data): Mark residualized vectors,
  bytevectors, and bitvectors as being read-only.