Remove a VEC from py-unwind.c

This removes a use of VEC from py-unwind.c, replacing it wit
std::vector.  It also changes saved_regs to hold a gdbpy_ref<>,
simplifying the memory management.

Tested against gdb.python on x86-64 Fedora 26.

gdb/ChangeLog
2018-06-27  Tom Tromey  <tom@tromey.com>

	* python/py-unwind.c (unwind_info_object) <saved_regs>: Now a
	std::vector.
	(unwind_infopy_str, pyuw_create_unwind_info)
	(unwind_infopy_add_saved_register, pyuw_sniffer)
	(unwind_infopy_dealloc, unwind_infopy_add_saved_register):
	Update.
	(struct saved_reg): Add constructor.
	<value>: Now a gdbpy_ref<>.