Add an `optional_vec` TLV format which makes sense, unlike vec_type

Add an `optional_vec` TLV format which makes sense, unlike vec_type

`vec_type` is confusing - it is happy to have a missing entry,
"reading" an empty `Vec` instead, but always writes something,
making a serialization round-trip different.

This is a problem for writing a new `Vec` which is
backwards-incompatible, but only if filled in. In that case we'd
really like the same read behavior, but not write anything if the
`Vec` is empty. Here we introduce such semantics via a new
`optional_vec` TLV format.