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/schema_compile_node.c b/src/schema_compile_node.c
index ed9d42c..1c33c60 100644
--- a/src/schema_compile_node.c
+++ b/src/schema_compile_node.c
@@ -2876,6 +2876,7 @@
                 /* inherit the module of the previous context node */
                 mod = ctx_node->module;
                 break;
+            case LY_VALUE_CANON:
             case LY_VALUE_XML:
                 /* not really defined */
                 LOGINT_RET(ctx->ctx);