Improve %string-dump and %symbol-dump

Improve %string-dump and %symbol-dump

%string-dump and %symbol-dump are modified to return assocation lists
of string and symbol attributes instead of printing to stderr.  They
are no longer conditional on SCM_DEBUG.

        * libguile/strings.c (scm_sys_string_dump)
        (scm_sys_symbol_dump): now returns alist of properties.  No longer
        require that SCM_DEBUG be defined.
        (scm_sys_stringbuf_hist): now conditional on
        SCM_STRING_LENGTH_HISTOGRAM

        * libguile/strings.h: scm_sys_string_dump and scm_sys_symbol dump
        are now declared as API