Remove free_memory_read_result_vector

Remove free_memory_read_result_vector

This changes read_memory_robust to return a std::vector, allowing the
removal of free_memory_read_result_vector and associated cleanups.
This patch also changes the functions it touches to be a bit more
robust with regards to deallocation; it's perhaps possible that
read_memory_robust could have leaked in some situations.

This patch is based on my earlier series to remove some MI cleanups.
Regression tested by the buildbot.

gdb/ChangeLog
2017-09-29  Tom Tromey  <tom@tromey.com>

	* target.c (read_whatever_is_readable): Change type of "result".
	Update.
	(free_memory_read_result_vector): Remove.
	(read_memory_robust): Change return type.  Update.
	* mi/mi-main.c (mi_cmd_data_read_memory_bytes): Update.  Use
	bin2hex, std::string.
	* target.h (memory_read_result_s): Remove typedef.
	(free_memory_read_result_vector): Remove.
	(read_memory_robust): Return std::vector.