#28 optional message fields

[Bench]

There are situations where it makes sense for messages
to have optional fields. RAMP is lacking a way to specify
these correctly.

It may not be practical to have optional integer fields
where 0 is a valid value, but for all other field types, the
field could be marked as "missing" by having an integer
value of zero, a collection size of zero, or a length of
zero. Even doing this, though, runs into problems with
the validation code of RAMP, because the validation has
to know that, as an optional field, a "zero" here is valid,
even though it normally wouldn't be.