Add support for pulling from remote archives

This necessitated a large set of changes.

We now support an "archive" mode for repositories.  In this mode,
files are stored "packed" rather than hard linked.  This allows one to
e.g. store an OSTree repository with root-owned files as non-root.  It
is also used as the basis for serving repositories via HTTP.

While doing this I realized that GVariant is endianness-dependent; I
decided to just store all data in big endian.

src/libostree/ostree-repo.h

@@ -53,17 +53,50 @@ OstreeRepo* ostree_repo_new (const char *path);
 
 gboolean      ostree_repo_check (OstreeRepo  *self, GError **error);
 
+const char *  ostree_repo_get_path (OstreeRepo  *self);
+
+gboolean      ostree_repo_is_archive (OstreeRepo  *self);
+
+GKeyFile *    ostree_repo_get_config (OstreeRepo *self);
+
+GKeyFile *    ostree_repo_copy_config (OstreeRepo *self);
+
+gboolean      ostree_repo_write_config (OstreeRepo *self,
+                                        GKeyFile   *new_config,
+                                        GError    **error);
+
 gboolean      ostree_repo_link_file (OstreeRepo *self,
-                                       const char   *path,
-                                       gboolean      ignore_exists,
-                                       gboolean      force,
-                                       GError      **error);
+                                     const char   *path,
+                                     gboolean      ignore_exists,
+                                     gboolean      force,
+                                     GError      **error);
+
+gboolean      ostree_repo_store_packfile (OstreeRepo       *self,
+                                           const char       *expected_checksum,
+                                           const char       *path,
+                                           OstreeObjectType  objtype,
+                                           GError          **error);
+
+gboolean      ostree_repo_store_object_trusted (OstreeRepo   *self,
+                                                const char   *path,
+                                                const char   *checksum,
+                                                OstreeObjectType objtype,
+                                                gboolean      ignore_exists,
+                                                gboolean      force,
+                                                gboolean     *did_exist,
+                                                GError      **error);
 
 gboolean      ostree_repo_resolve_rev (OstreeRepo  *self,
                                        const char  *rev,
                                        char       **out_resolved,
                                        GError     **error);
 
+gboolean      ostree_repo_write_ref (OstreeRepo  *self,
+                                     gboolean     is_local,
+                                     const char  *name,
+                                     const char  *rev,
+                                     GError     **error);
+
 gboolean      ostree_repo_load_variant (OstreeRepo *self,
                                           const char   *sha256,
                                           OstreeSerializedVariantType *out_type,