Initial basic static delta code drop

This has a very basic level of functionality (deltas can be generated,
and applied offline).  There is only some stubbed out pull code to
fetch them via HTTP.

But, better to commit this now and improve it from a known starting
point, rather than have it languish in a branch.

src/libotutil/ot-variant-utils.h

@@ -46,6 +46,13 @@ gboolean ot_util_variant_map (GFile *src,
                               GVariant **out_variant,
                               GError  **error);
 
+gboolean ot_util_variant_map_fd (GFileDescriptorBased *stream,
+                                 goffset              offset,
+                                 const GVariantType  *type,
+                                 gboolean             trusted,
+                                 GVariant           **out_variant,
+                                 GError             **error);
+
 gboolean ot_util_variant_from_stream (GInputStream         *src,
                                       const GVariantType   *type,
                                       gboolean              trusted,
@@ -58,5 +65,10 @@ GInputStream *ot_variant_read (GVariant             *variant);
 GVariantBuilder *ot_util_variant_builder_from_variant (GVariant            *variant,
                                                        const GVariantType  *type);
 
+GVariant *
+ot_variant_new_from_bytes (const GVariantType  *type,
+                           GBytes        *bytes,
+                           gboolean       trusted);
+
 G_END_DECLS