Place displaced step data directly in inferior structure

Place displaced step data directly in inferior structure

This patch moves the per-inferior data related to displaced stepping to
be directly in the inferior structure, rather than in a container on the
side.

On notable difference is that previously, we deleted the state on
inferior exit, which guaranteed a clean state if re-using the inferior
for a new run or attach.  We now need to reset the state manually.

At the same time, I changed step_saved_copy to be a gdb::byte_vector, so
it is automatically freed on destruction (which should plug the leak
reported here [1]).

[1] https://sourceware.org/ml/gdb-patches/2018-11/msg00202.html

gdb/ChangeLog:

	* inferior.h (class inferior) <displaced_step_state>: New field.
	* infrun.h (struct displaced_step_state): Move here from
	infrun.c.  Initialize fields, add constructor.
	<inf>: Remove field.
	<reset>: New method.
	* infrun.c (struct displaced_step_inferior_state): Move to
	infrun.h.
	(displaced_step_inferior_states): Remove.
	(get_displaced_stepping_state): Adust.
	(displaced_step_in_progress_any_inferior): Adjust.
	(displaced_step_in_progress_thread): Adjust.
	(displaced_step_in_progress): Adjust.
	(add_displaced_stepping_state): Remove.
	(get_displaced_step_closure_by_addr): Adjust.
	(remove_displaced_stepping_state): Remove.
	(infrun_inferior_exit): Call displaced_step_state.reset.
	(use_displaced_stepping): Don't check for NULL.
	(displaced_step_prepare_throw): Call
	get_displaced_stepping_state.
	(displaced_step_fixup): Don't check for NULL.
	(prepare_for_detach): Don't check for NULL.