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.c b/src/tree_schema.c
index 4377cfd..e58e26b 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -129,6 +129,12 @@
     [LY_STMT_YIN_ELEMENT] = {"yin-element", "value", STMT_FLAG_ID},
 };
 
+API const char *
+ly_stmt2str(enum ly_stmt stmt)
+{
+    return stmt_attr_info[stmt].name;
+}
+
 const char * const ly_devmod_list[] = {
     [LYS_DEV_NOT_SUPPORTED] = "not-supported",
     [LYS_DEV_ADD] = "add",
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.
diff --git a/src/tree_schema_internal.h b/src/tree_schema_internal.h
index 6c13882..cdc478f 100644
--- a/src/tree_schema_internal.h
+++ b/src/tree_schema_internal.h
@@ -48,9 +48,6 @@
 /* statements informations filled in tree_schema.c */
 extern struct stmt_info_s stmt_attr_info[];
 
-/* list of the YANG statements strings */
-#define ly_stmt2str(STMT) stmt_attr_info[STMT].name
-
 /* list of the deviate modifications strings */
 extern const char * const ly_devmod_list[];
 #define ly_devmod2str(TYPE) ly_devmod_list[TYPE]