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/path.c b/src/path.c
index 1fbe07b..7ca8616 100644
--- a/src/path.c
+++ b/src/path.c
@@ -451,6 +451,7 @@
             /* inherit module of the previous node */
             *mod = prev_ctx_node->module;
             break;
+        case LY_VALUE_CANON:
         case LY_VALUE_XML:
             /* not really defined */
             LOGINT_RET(ctx);