Add new ex history command

This is the rpm-ostree equivalent of `dnf history`. As opposed to the
history of the refspec (i.e. `ostree log`), this shows the history of
the system, i.e. the refspecs the host deployed, checksums, versions,
layered packages, etc... The amount of details remembered is similar to
what shows up in `status`.

There's definitely some further enhancements possible (e.g. printing
package diffs, displaying rollbacks), though this seems in good enough
shape as a first cut.

Closes: coreos#1489

rust/cbindgen.toml

@@ -3,6 +3,8 @@ language = "C"
 header = "#pragma once\n#include <gio/gio.h>\ntypedef GError RORGError;\ntypedef GHashTable RORGHashTable;"
 trailer = """
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(RORTreefile, ror_treefile_free)
+G_DEFINE_AUTOPTR_CLEANUP_FUNC(RORHistoryCtx, ror_history_ctx_free)
+G_DEFINE_AUTO_CLEANUP_CLEAR_FUNC(RORHistoryEntry, ror_history_entry_clear)
 """