Also move most internal vector representation into a private header
file.
* libguile/vectors-internal.h: New file.
* libguile/vectors.h (SCM_I_IS_MUTABLE_VECTOR, SCM_I_VECTOR_WELTS)
(SCM_I_VECTOR_ELTS, SCM_I_VECTOR_LENGTH): Remove these internal
definitions.
* libguile/vectors.c: Adapt to use new data types.
* libguile/eval.c: Include internal file.
* libguile/init.c: Include internal file.
* libguile/array-handle.c (scm_array_get_handle): Use new functions.
