schema CHANGE move ly_stmt2str() into public API

Useful function to work with the statements enumeration. However to
hide the internal structure holding the information about the
statements, change it from macro to a function.
diff --git a/src/tree_schema.h b/src/tree_schema.h
index 142a33d..02e932c 100644
--- a/src/tree_schema.h
+++ b/src/tree_schema.h
@@ -442,6 +442,13 @@
 const char *lyext_parent2str(LYEXT_PARENT type);
 
 /**
+ * @brief Stringify statement identifier.
+ * @param[in] stmt The statement identifier to stringify.
+ * @return Constant string representation of the given @p stmt.
+ */
+const char *ly_stmt2str(enum ly_stmt stmt);
+
+/**
  * @brief Convert nodetype to statement identifier
  * @param[in] nodetype Nodetype to convert.
  * @return Statement identifier representing the given @p nodetype.