gdb/python: implement DisassemblerResult.__str__ method

Add the DisassemblerResult.__str__ method.  This gives the same result
as the DisassemblerResult.string attribute, but can be useful
sometimes depending on how the user is trying to print the object.

There's a test for the new functionality.

gdb/python/py-disasm.c

@@ -608,6 +608,21 @@ gdbpy_disassembler::read_memory_func (bfd_vma memaddr, gdb_byte *buff,
   return 0;
 }
 
+/* Implement __str__ for the DisassemblerResult type.  */
+
+static PyObject *
+disasmpy_result_str (PyObject *self)
+{
+  disasm_result_object *obj = (disasm_result_object *) self;
+
+  gdb_assert (obj->content != nullptr);
+  gdb_assert (obj->content->size () > 0);
+  gdb_assert (obj->length > 0);
+  return PyUnicode_Decode (obj->content->c_str (),
+			   obj->content->size (),
+			   host_charset (), nullptr);
+}
+
 /* Implement DisassemblerResult.length attribute, return the length of the
    disassembled instruction.  */
 
@@ -624,14 +639,7 @@ disasmpy_result_length (PyObject *self, void *closure)
 static PyObject *
 disasmpy_result_string (PyObject *self, void *closure)
 {
-  disasm_result_object *obj = (disasm_result_object *) self;
-
-  gdb_assert (obj->content != nullptr);
-  gdb_assert (obj->content->size () > 0);
-  gdb_assert (obj->length > 0);
-  return PyUnicode_Decode (obj->content->c_str (),
-			   obj->content->size (),
-			   host_charset (), nullptr);
+  return disasmpy_result_str (self);
 }
 
 /* Implement DisassemblerResult.__init__.  Takes two arguments, an
@@ -1147,7 +1155,7 @@ PyTypeObject disasm_result_object_type = {
   0,						/*tp_as_mapping*/
   0,						/*tp_hash */
   0,						/*tp_call*/
-  0,						/*tp_str*/
+  disasmpy_result_str,				/*tp_str*/
   0,						/*tp_getattro*/
   0,						/*tp_setattro*/
   0,						/*tp_as_buffer*/