schema REFACTOR rename argument member in extension definition

The argname better describes meaning of the data and avoids conflicts
with the argument member in extension instances. The change affects both
the parsed and compiled extension definition structures (lysp_ext and
lysc_ext)
diff --git a/src/tree_schema_free.c b/src/tree_schema_free.c
index 89a5612..cea7784 100644
--- a/src/tree_schema_free.c
+++ b/src/tree_schema_free.c
@@ -119,7 +119,7 @@
 lysp_ext_free(struct ly_ctx *ctx, struct lysp_ext *ext)
 {
     lydict_remove(ctx, ext->name);
-    lydict_remove(ctx, ext->argument);
+    lydict_remove(ctx, ext->argname);
     lydict_remove(ctx, ext->dsc);
     lydict_remove(ctx, ext->ref);
     FREE_ARRAY(ctx, ext->exts, lysp_ext_instance_free);
@@ -502,7 +502,7 @@
         return;
     }
     lydict_remove(ctx, (*ext)->name);
-    lydict_remove(ctx, (*ext)->argument);
+    lydict_remove(ctx, (*ext)->argname);
     FREE_ARRAY(ctx, (*ext)->exts, lysc_ext_instance_free);
     free(*ext);
     *ext = NULL;