Class-ify macro_buffer

Class-ify macro_buffer

This patch changes macro_buffer to be a bit more of a C++ class,
adding constructors, a destructor, and some members.  Then this is
used to remove various cleanups in macroexp.c.

2018-02-08  Tom Tromey  <tom@tromey.com>

	* macroexp.c (struct macro_buffer): Add initializers for some
	members.
	(init_buffer, init_shared_buffer, free_buffer)
	(free_buffer_return_text): Remove.
	(macro_buffer): New constructors.
	(~macro_buffer): New destructor.
	(macro_buffer::set_shared): New method.
	(macro_buffer::resize_buffer, macro_buffer::appendc)
	(macro_buffer::appendmem): Now methods, not free functions.
	(set_token, append_tokens_without_splicing, stringify)
	(macro_stringify): Update.
	(gather_arguments): Change return type.  Remove argc_p argument,
	add args_ptr argument.  Use std::vector.
	(substitute_args): Remove argc argument.  Accept std::vector.
	(expand): Update.  Use std::vector.
	(scan, macro_expand, macro_expand_next): Update.