Give access to type size info without explicit use of dict_attr_sizes[]

Give access to type size info without explicit use of dict_attr_sizes[]

The change provides min_size() and max_size() macros and a Boolean
function, is_fixed_size(). All take an fr_type_t value and do the
subscripting of dict_attr_sizes for the user. References to
dict_attr_sizes[] other than definition and initialization have
been changed to use these, to exercise them and I believe make
the intent of the rewritten code clearer.