Returns a list of 'field-info' structures. If 'include-invisible-p'
is set to true, fields declared as invisible will be returned as
well.

If 'expand-mixins' is set to true (default), mixin fields will be
expanded into inline fields, and will not be present in the results.

If 'custom-fields' is not null, it is expected to be a list with each
element being either a custom field, or a cons cells. If an element is
a custom field, the field is returned at the end of other fields. If
an element is a cons cell, CAR is expected to be a positive integer,
and CDR is expected to be a custom-field. In this case the
custom-field is inserted before the element with the index in the CAR
of the cons cell.

Each custom field can be either a field-info structure or a
view-field. Field-info structures are inserted as is, and view-fields
are wrapped in field-info structures with common-sense defaults.