[triton/ast] renamed ast -> lang in namespace and file structure

lib/lang/module.cpp

@@ -0,0 +1,18 @@
+#include "triton/lang/module.h"
+#include "triton/ir/module.h"
+
+
+namespace triton{
+
+namespace lang{
+
+/* Translation unit */
+ir::value* translation_unit::codegen(ir::module *mod) const{
+  mod->add_new_scope();
+  decls_.codegen(mod);
+  return nullptr;
+}
+
+}
+
+}