Calls the given function for each data element of the datalist.
The function is called with each data element's Quark id and data, together with the given
user_data parameter. Note that this function is NOT thread-safe. So unless datalist can be protected from any
modifications during invocation of this function, it should not be called.
| func |
the function to call for each data element. |
| datalist |
a datalist. |
| user_data |
user data to pass to the function. |