Fixtype of length suffixes [C:6.4.4.1] [C:A.1.5].
This is a tagged union type, introduced by fty::deftagsum.
This corresponds to long-suffix and long-long-suffix
in the grammar in [C].
We use the term `length suffix' here,
but the grammar in [C] does not use that term,
although it is arguably a good term to denote
both long and long-long suffixes.
This captures the four possible suffixes