lib/ref: Add OstreeCollectionRef type for globally unique refs

This is a type representing the tuple (collection ID, ref name), which is
guaranteed to be globally unique. It will be used in upcoming commits.

It introduces the concept of a ‘collection’ which is a unique, curated
set of refs which lie in the same trust domain (i.e. all signed by the
same key and validated by the same developer). Flathub might be a
collection, for example; or the set of OS refs coming from a particular
OS vendor.

It includes a function for validating collection IDs.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #924
Approved by: cgwalters

apidoc/ostree-experimental-sections.txt

@@ -1,3 +1,18 @@
+<SECTION>
+<FILE>ostree-ref</FILE>
+OstreeCollectionRef
+ostree_collection_ref_new
+ostree_collection_ref_dup
+ostree_collection_ref_free
+ostree_collection_ref_hash
+ostree_collection_ref_equal
+OstreeCollectionRefv
+ostree_collection_ref_dupv
+ostree_collection_ref_freev
+<SUBSECTION Standard>
+ostree_collection_ref_get_type
+</SECTION>
+
 <SECTION>
 <FILE>ostree-remote</FILE>
 OstreeRemote
@@ -5,3 +20,8 @@ ostree_remote_ref
 ostree_remote_unref
 ostree_remote_get_name
 </SECTION>
+
+<SECTION>
+<FILE>ostree-misc-experimental</FILE>
+ostree_validate_collection_id
+</SECTION>