Use std::vector for mi_ui_out_data::streams

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

gdb/ChangeLog:

	* mi/mi-out.c: Remove vec.h include.
	(mi_ui_out_data) <streams>: Change type to std::vector.
	(mi_field_string): Update.
	(mi_field_fmt): Update.
	(mi_flush): Update.
	(mi_redirect): Update.
	(field_separator): Update.
	(mi_open): Update.
	(mi_close): Update.
	(mi_out_buffered): Update.
	(mi_out_rewind): Update.
	(mi_out_put): Update.
	(mi_out_data_ctor): Update.
	(mi_out_data_dtor): Don't free streams.