libtextstyle: Add is_instance_of_* predicates.

libtextstyle: Add is_instance_of_* predicates.

* libtextstyle/gnulib-local/lib/fd-ostream.oo.h (is_instance_of_fd_ostream): New
declaration.
* libtextstyle/gnulib-local/lib/fd-ostream.oo.c (is_instance_of_fd_ostream): New
function.
* libtextstyle/gnulib-local/lib/file-ostream.oo.h (is_instance_of_file_ostream):
New declaration.
* libtextstyle/gnulib-local/lib/file-ostream.oo.c (is_instance_of_file_ostream):
New function.
* libtextstyle/gnulib-local/lib/html-ostream.oo.h (is_instance_of_html_ostream):
New declaration.
* libtextstyle/gnulib-local/lib/html-ostream.oo.c (is_instance_of_html_ostream):
New function.
* libtextstyle/gnulib-local/lib/html-styled-ostream.oo.h
(is_instance_of_html_styled_ostream): New declaration.
* libtextstyle/gnulib-local/lib/html-styled-ostream.oo.c
(is_instance_of_html_styled_ostream): New function.
* libtextstyle/gnulib-local/lib/iconv-ostream.oo.h
(is_instance_of_iconv_ostream): New declaration.
* libtextstyle/gnulib-local/lib/iconv-ostream.oo.c
(is_instance_of_iconv_ostream): New function.
* libtextstyle/gnulib-local/lib/memory-ostream.oo.h
(is_instance_of_memory_ostream): New declaration.
* libtextstyle/gnulib-local/lib/memory-ostream.oo.c
(is_instance_of_memory_ostream): New function.
* libtextstyle/gnulib-local/lib/noop-styled-ostream.oo.h
(is_instance_of_noop_styled_ostream): New declaration.
* libtextstyle/gnulib-local/lib/noop-styled-ostream.oo.c
(is_instance_of_noop_styled_ostream): New function.
* libtextstyle/gnulib-local/lib/styled-ostream.oo.h
(is_instance_of_styled_ostream): New declaration.
* libtextstyle/gnulib-local/lib/styled-ostream.oo.c
(is_instance_of_styled_ostream): New function.
* libtextstyle/gnulib-local/lib/term-ostream.oo.h (is_instance_of_term_ostream):
New declaration.
* libtextstyle/gnulib-local/lib/term-ostream.oo.c (is_instance_of_term_ostream):
New function.
* libtextstyle/gnulib-local/lib/term-styled-ostream.oo.h
(is_instance_of_term_styled_ostream): New declaration.
* libtextstyle/gnulib-local/lib/term-styled-ostream.oo.c
(is_instance_of_term_styled_ostream): New function.
* libtextstyle/lib/textstyle.h (is_instance_of_styled_ostream,
is_instance_of_file_ostream, is_instance_of_fd_ostream,
is_instance_of_term_ostream, is_instance_of_memory_ostream,
is_instance_of_iconv_ostream, is_instance_of_html_ostream,
is_instance_of_term_styled_ostream, is_instance_of_html_styled_ostream,
is_instance_of_noop_styled_ostream): New declarations.
* libtextstyle/tests/test-instanceof.c: New file.
* libtextstyle/tests/Makefile.am: Arrange to compile it.
* libtextstyle/doc/libtextstyle.texi (The output stream hierarchy): Mention the
is_instance_of_* predicates.
* libtextstyle/NEWS: Mention the new functions.