Return std::string from canonical_to_fullform

Return std::string from canonical_to_fullform

This changes canonical_to_fullform to return a std::string, and
changes decode_line_2 to use std::vector.  This allows for the removal
of some cleanups.

ChangeLog
2018-04-05  Tom Tromey  <tom@tromey.com>

	* linespec.c (canonical_to_fullform): Return std::string.
	(filter_results): Update.
	(struct decode_line_2_item): Add constructor.
	<fullform, displayform>: Now std::string.
	(decode_line_2_compare_items): Now a std::sort comparator.
	(decode_line_2): Update.