libyang CHANGE add LY_VALUE_CANON

Add a separate value format of the canonical representation of the type.
The print callback API of the type plugins was modified to reflect the
need of creating canonical representation of the value.
diff --git a/src/tree.h b/src/tree.h
index f573504..eb0fd71 100644
--- a/src/tree.h
+++ b/src/tree.h
@@ -233,6 +233,7 @@
  * @brief All kinds of supported value formats and prefix mappings to modules.
  */
 typedef enum {
+    LY_VALUE_CANON,           /**< canonical value, prefix mapping is type-specific */
     LY_VALUE_SCHEMA,          /**< YANG schema value, prefixes map to YANG import prefixes */
     LY_VALUE_SCHEMA_RESOLVED, /**< resolved YANG schema value, prefixes map to module structures directly */
     LY_VALUE_XML,             /**< XML data value, prefixes map to XML namespace prefixes */