Change value history to use value_ref_ptr

Change value history to use value_ref_ptr

This simplifies the value history implementation by replacing the
current data structure with a std::vector, and by making the value
history simply hold a reference to each value.

gdb/ChangeLog
2018-04-06  Tom Tromey  <tom@tromey.com>

	* value.c (VALUE_HISTORY_CHUNK, struct value_history_chunk)
	(value_history_chain, value_history_count): Remove.
	(value_history): New global.
	(record_latest_value, access_value_history, show_values)
	(preserve_values): Update.