MASt doesn't support the 'allowed_values_function' property of list fields

The Field API submodule for defining the list field types allows for the definition of the set of allowed values based on a function. There's a function that returns the allowed values regardless of whether they've been set in the 'allowed_values' setting in the field or are generated by a function. MASt should be using this function, otherwise it cannot retrieve the allowed values for fields that use it.