Use std::vector for cli_ui_out_data::streams

Use a standard vector instead of the home-made version.  I used a vector
of plain pointers, because the cli_ui_out_data object doesn't own the
streams objects (i.e. they shouldn't be deleted when the vector is
deleted).

gdb/ChangeLog:

	* cli-out.h (cli_ui_out_data) <streams>: Change type to
	std::vector.
	* cli-out.c: Remove vec.h include.
	(cli_uiout_dtor): Update.
	(cli_field_fmt): Update.
	(cli_spaces): Update.
	(cli_text): Update.
	(cli_message): Update.
	(cli_flush): Update.
	(cli_redirect): Update.
	(out_field_fmt): Update.
	(field_separator): Update.
	(cli_out_data_ctor): Update.
	(cli_out_new): Update.
	(cli_out_set_stream): Update.