More C++-ification for struct display

More C++-ification for struct display

This changes displays to have a constructor, use bool and std::string,
and to be stored using std::vector.  The ALL_DISPLAYS and
ALL_DISPLAYS_SAFE macros are removed.  While internal iteration is
still done via map_display_numbers, this is updated to use a
function_view.  These changes simplify the code somewhat; for example,
free_display can now be removed in favor of ordinary destruction.

gdb/ChangeLog
2020-05-08  Tom Tromey  <tom@tromey.com>

	* printcmd.c (struct display) <next>: Remove.
	<display>: New constructor.
	<exp_string>: Now a std::string.
	<enabled_p>: Now a bool.
	(display_number): Move definition earlier.
	(displays): Rename from display_chain.  Now a std::vector.
	(ALL_DISPLAYS, ALL_DISPLAYS_SAFE): Remove.
	(display_command): Update.
	(do_one_display, disable_display)
	(enable_disable_display_command, do_enable_disable_display):
	Update.
	(free_display): Remove.
	(clear_displays): Rewrite.
	(delete_display): Update.
	(map_display_numbers): Use function_view.  Remove "data"
	parameter.  Update.
	(do_delete_display): Remove.
	(undisplay_command): Update.
	(do_one_display, do_displays, disable_display)
	(info_display_command): Update.
	(do_enable_disable_display): Remove.
	(enable_disable_display_command)
	(clear_dangling_display_expressions): Update.